INTERFACIAL APPLICATIONS IN ENVIRONMENTAL ENGINEERING - CHAPTER 17 (END) docx

Thereare many research papers on the catalytic dechlorination of various organic chlo-rine compounds [25–26], but most of them deal with the removal of chlorine bynoble metal catalysts i

The widespread use of plastics in modern industry has resulted in a huge volume

of waste plastic requiring treatment In the years between 1975 and 1998 in Japan,the amount of postconsumer plastic waste increased 3.8 times, plastic production2.7 times, and consumption of plastics 3.2 times(Table 1)[1] It can be antici-pated that this trend will continue unless more intensive recycling of plastic waste

is undertaken In the past, landfilling and incineration (without energy recovery)were widely practiced in many countries, but these options have been criticized

in recent years for their adverse effects on the environment Not only do thewaste plastics make the environment unsightly, but also their energy and chemicalcontent are lost Presently, the alternative options for the treatment of plasticwaste are: (1) mechanical recycling, (2) energy recycling, and (3) feedstock orchemical recycling Mechanical recycling by melting and remolding of the usedplastics is limited to particular types of polymers and applications of the recycledmaterials If mechanical recycle is not feasible, energy recovery by incineration

is another option, since the calorific value of plastics is very high However,incineration may lead to the formation of pollutants, such as dioxins and othertoxins, depending on the composition of the plastic waste and the nature of com-bustion Feedstock or chemical recycling of plastic waste by pyrolysis or thermaldegradation permits the recovery of valuable hydrocarbons, which can be used

as feedstock materials or fuels The plastic wastes are transformed into usefulchemicals by thermal degradation in an inert atmosphere Generally the gaseousand liquid products obtained are complex mixtures of hydrocarbons and otherorganic compounds, whose composition depends on the composition of the plas-

* Current affiliation: The University of Newcastle, Callghan, New South Wales, Australia.

TABLE 1 Production, Consumption, and Disposal Rates of Polymer Resins in Japan

1975 1998 IncreaseResin production 517 1391 2.7 timesDomestic consumption of plastic goods 315 1020 3.2 timesDisposal of plastic waste 261 984 3.8 times

tic waste The potentials and prospects of chemical recycling of plastic wastehave been addressed in a recent review [2]

II DEHALOGENATION ISSUES IN THE CHEMICAL

RECYCLING OF PLASTIC WASTE

Most of the practical research on plastic degradation to fuel oil is limited to PE,

PP, and PS but excludes PVC, since it contains chlorine (56 wt%) and releasestoxic and corrosive hydrogen chloride gas during the early stage of degradation

at 280–320°C Upon dehydrochlorination, PVC forms a polyene macromolecularstructure and it decomposes at higher temperatures (380–600°C) to produce vola-tiles containing aliphatic, olefinic, and aromatic hydrocarbons and solid chars[3] Dehydrochlorination of PVC has been studied extensively, in particular thekinetics and mechanism of PVC degradation [4–9] Although more than 99% ofthe chlorine content of PVC is removed as HCl in the early stages, the remainingchlorine in the polyene macromolecular structure may lead to the formation ofunwanted Cl-containing organics Detailed studies on the identification of theproducts evolved from PVC degradation and their mechanisms of formation havealso been reported [10–12] We have reported that various types of organic chlo-rine compounds are produced when PVC mixed with PE, PP, and PS is thermallydegraded at 430°C, and we have clarified the route of formation of these com-pounds [13–14] Hydrogen chloride released from PVC degradation reacts withthe hydrocarbons produced from the other polymers to form organic chlorinecompounds Generally, municipal plastic waves (MPW) contain all kinds of plas-tics, including PVC In Japan, municipal plastic wastes contain about 5–10 wt%PVC [15] Therefore, thermal degradation of MPW produces unwanted organicchlorine compounds in the oil The waste plastic–derived oil that contains organicchlorine compounds cannot be used safely as a fuel oil because there is a highpossibility of producing toxic compounds, such as dioxins, dibenzofurans, andbiphenyls, during combustion It is necessary to remove the organic chlorinecompounds from the oil before use

Halogens other than chlorine may also be present in the polymer as additives,for instance, brominated flame-retardant compounds, such as polybrominated

Catalytic Dehalogenation of Plastic-Derived Oil 331

benzene compounds, are added to PS and ABS to inhibit or modify polymercombustion when heated in an oxidative atmosphere Electrical and electronichome appliances are made of high-impact polystyrene (HIPS) containing bromi-nated flame retardants Some of these brominated plastics end up in municipalplastic wastes [16] Thus debromination of brominated hydrocarbons in the plas-tic-derived oil is another important issue Dehalogenation of the plastic-derivedoil is a key technology for the success of chemical recycling of mixed plasticscontaining chlorinated and brominated polymers into fuel oil or chemicals Com-pared to PVC, very little work has been reported on the pyrolysis of brominatedflame retardants containing plastic waste The separation of halogenated flameretardants from polymer matrixes with extraction using supercritical fluids such

as supercritical carbon dioxide (SC-CO2) has been studied [17] ior and degradation-mechanism studies on brominated polystyrenes have beenstudied mainly by thermogravimetry, thermal volatilization analysis (TVA), andPy-GC/MS technique and show that the degradation occurs mainly to the mono-mer via radical polymerization [18] We have reported the formation of bromi-nated hydrocarbons when brominated-flame-retardant-containing HIPS and itsmixture with other plastics are thermally degraded to obtain fuel oil [19]

Thermal-behav-III OUR APPROACH TO THE DEHALOGENATION

OF PLASTIC-DERIVED OIL

Various methods are used to remove the chlorine by coprocessing catalysts orabsorbent during the pyrolysis [20–22] All of these efforts succeed to someextent in removing chlorine content in PVC-containing plastics before or duringthe pyrolysis However, even the presence of a small amount of chlorine (⬍1%)leads to the formation of chloro-organic compounds We observed that even whenthe PVC is dechlorinated to⬎98%, chloro-organic compounds are still produced[23] There has been no detailed study of the removal of chloro-organic com-pounds from the plastic-derived oil thus far

Catalytic dechlorination is a promising method for the removal of organicchlorine compounds, compared to other methods, such as combustion [24] Thereare many research papers on the catalytic dechlorination of various organic chlo-rine compounds [25–26], but most of them deal with the removal of chlorine bynoble metal catalysts in the presence of hydrogen We have been developing ironoxide–carbon composite catalysts for the dehydrochlorination and dehydrobrom-ination of plastic-derived oil In our proposed catalytic process, waste plastic–derived oil containing chlorinated and brominated hydrocarbons is treated oversolid catalysts in a fixed-bed flow-type reactor in a temperature range of 300–

400°C at atmospheric pressure in He or N2flow in order to decompose the nated hydrocarbons into hydrocarbons and hydrogen halides We have demon-

haloge-strated the effectiveness of our catalysts for the dehydrodechlorination of modelcompounds, such as chlorocyclohexane and 1-chloroheptane, in a fixed-bed reac-tor These iron oxides are reported to be effective in the destruction or reduction

of dioxins from municipal solid-waste incineration products [27] In ABS dation, iron oxides showed a catalytic effect in decomposition of N-containingheterocyclic compounds from the degradation oil [28–29] This review summa-rizes our recent studies on (1) the catalytic dechlorination of PVC-containingmixed plastics–derived oil [30], (2) municipal waste plastic–derived oil [31], and(3) simultaneous dechlorination and debromination from the pyrolysis products

degra-of PVC and brominated flame-retardant-containing high-impact polystyrene(HIPS) mixed plastics over iron-base catalysts

A Catalytic Dechlorination of Chloro-Organic

Compounds from PVC-Containing Mixed

Plastic-Derived Oil over Iron Oxide Catalysts

We have reported the catalytic activity of various iron oxide and iron oxide–carbon composite catalysts for the dechlorination of chloro-organic compoundsformed during the thermal degradation of PVC-containing mixed plastics PVC-containing mixed plastic-derived oil was prepared by thermal degradation in aseparate facility, and the dechlorination of the derived oil was performed in afixed-bed flow-type reactor Emphasis was put on the stability of iron oxide–based catalysts in the presence of HCl gas produced during dechlorination ofmixed plastic–derived oil The PVC-containing waste mixed plastic (MX/PVC)–derived oil was prepared by degrading mixed plastics containing PE (33%), PP(33%), PS (33%), and PVC (1%) as a model sample at 410°C Details of theexperimental procedure for the preparation of mixed plastic (MX/PVC)–derivedoil is given elsewhere [32] Toda Kogyo Corporation, Japan, supplied the cata-lystsα-Fe2O3[PDC-03 (2)],γ-Fe2O3(TR99701), and iron oxide–carbon compos-ite (TR97305 and TR99300) catalysts used in this study TR97305 and TR99300were prepared from physical mixtures of iron oxide (Goethite: FeOOH) and phe-nol resins in a ratio of 9 : 1 by heat treatment After the heat treatment at 500°C

in N2flow, the product catalysts were identified as Fe3O4and carbon composites.The physical characteristics of the catalysts used in this study are presented inTable 2 TR97305 and TR99300 are two iron oxide–carbon composites withsimilar composition but prepared by different methods These composite catalystswere prepared in order to increase the physical strength of the catalyst pellets.The dechlorination of mixed plastic–derived oil was carried out using a fixed-bed reactor at atmospheric pressure with a reaction temperature of 350°C In atypical experiment, about 1 mL (0.1-mm average size) of the catalyst was loaded

in between two quartz wool beds and treated in He atmosphere (60 cc/min) atreaction temperature for 1 h before feeding the mixed plastic–derived oil (10

Catalytic Dehalogenation of Plastic-Derived Oil 333

TABLE 2 Physical Characteristics of Iron Oxide and Iron Oxide–Carbon CompositeCatalysts

Surface area Iron oxide CarbonCatalyst (m2-g⫺1) content (wt%) content (wt%) XRD

chromatogra-mm⫻ 0.17 µm) using 1, 2, 4-trichlorobenzene as an internal standard.The conversion by the catalysts in the dechlorination of the mixed plastic–derived oil was calculated as follows:

[Cl content in mixed plastic–derived oil–Cl content in product]

[Cl content in mixed plastic–derived oil] (1)The physicochemical properties of the MX/PVC-derived oil estimated by stan-dard procedures and are tabulated inTable 3.The oil derived from mixed plastic-degradation oil contained 1894 ppm of organic chlorine compounds The carbonnumber distribution of all compounds (C-NP gram) in MX/PVC derived oil andthe carbon number distribution of chloro-organic compounds (Cl-NP gram) areshown inFigure 1aand 1b, respectively The C-NP gram was obtained by plottingthe weight percent of carbon-containing compounds in the MX/PVC oil againstthe carbon number of equivalent b.p of normal paraffin The Cl-NP gram wasalso obtained by plotting the content of Cl-containing compounds in the MX/PVC oil against the carbon number of each normal paraffin (equivalent to boilingpoints) [33] The chlorine compounds were distributed mainly in the b.p range

of nC6 to nC19 The main organic chloro compounds are in the range of nC 6–

nC11 The major organic chlorine compounds were identified as 2-chloro,2-methyl-propane, 2-chloro,2-methyl pentane, chloroethyl benzene, and 2-chloro,2-phenyl propane [13]

TABLE 3 Physicochemical Properties of MX/

Conradson carbon residue (wt%) 0.11

Calorific value (cal/g) 10600

Catalytic Dehalogenation of Plastic-Derived Oil 335

FIG 1 (a) Carbon number distribution of all compounds (C-NP gram) in MX/PVCderived oil and (b) carbon number distribution of chlorine organic compounds (Cl-NPgram)

produced HCl, and that this iron chloride phase is also active for the tion of chloro-organic compounds

dechlorina-Based on the foregoing results, it is anticipated that the physically adsorbedHCl might be responsible for the initial decrease in activity We carried out anexperiment where the reaction was stopped after 10 h onstream; later, the catalystwas treated in He for 1 h at reaction temperature These results are shown inFigure 5.After the first He treatment, the catalyst regained most of its activity.This behavior was found after this procedure was repeated twice These resultssuggest that a continuous removal of reversible adsorbed HCl from the catalystsurface will be necessary to maintain a stable dechlorination activity of the cata-lyst Further experiments were carried out using He as a carrier gas (5 mL/cm3)over iron oxidesα- and γ-Fe2O3and iron oxide–carbon composites TR97305 andTR97300 at a liquid hourly space velocity (LHSV) of 40 h1 Because the true

FIG 2 MX/PVC-derived oil dechlorination activity over TR97305 catalyst with time

in the absence of carrier gas

value of gas hourly space velocity (GHSV) is difficult to determine due to thepresence of a large number of hydrocarbons, only the LHSV, without considering

He carrier flow rate, was used; the results are presented inFigure 6 The ironoxides and iron oxide–carbon composite showed a high activity in dechlorination.These catalysts showed similar high activity in the dechlorination of modelchloroalkane, like chlorocyclohexane and 1-chloroheptane [37] The effect ofLHSV overγ-Fe2O3catalyst was studied; the results are presented inFigure 7.The removal of chlorine compounds at lower space felocity was very high None

of the catalysts studied in this reaction greatly affected the carbon number bution (C-NP gram) during the dechlorination It is reasonable to think that theremoval of chlorine may result in a change in the carbon number (equivalent tothe b.p of normal paraffin) distribution However, the change in carbon numberdistribution was negligible, since the amount of chlorine-containing compounds

distri-in the origdistri-inal mixed plastics–derived oil was not high enough to produce anysignificant change in the C-NP gram

Figure 8shows the time-on-stream analysis over TR97305 and TR99300 lysts It is worth mentioning that these catalysts are very active and that no appre-ciable deactivation was observed in 24 h of time on stream It has also reported

cata-on a Ni/SiO2catalyst system that a rapid deactivation (60%) occurred within 4

h during the dechlorination of chloro alkanes in the presence of He carrier gas,but the catalyst deactivation was suppressed by hydrogen [38] It is well knownthat the suppression of catalyst deactivation in the presence of H2 is due to adisplacement of the hydrogen halide by dissociated hydrogen, which acts to cleanthe surface of the metal catalyst [40] Surprisingly, in the present iron oxidecatalyst system, in the presence of He carrier, high dechlorination activity withhigher stability was achieved without using any hydrogen atmosphere

Catalytic Dehalogenation of Plastic-Derived Oil 337

FIG 3 X-ray diffraction patterns of the (a) fresh and (b) used TR97305 catalyst (䊉)

Fe3O4; (䉱) FeCl2⋅ 4H2O

B Removal of Organic Chlorine Compounds from

Municipal Waste Plastic–Derived Oil by Catalytic

Dehydrochlorination over Iron Oxides (Fe3O4

-Carbon), Zinc Oxide, Magnesium Oxide, and Red

FIG 4 Activity patterns of TR97305 catalysts before and after HCl treatment in thedechlorination of MX/PVC-derived oil (䊊) Fresh catalyst; (■) HCl-treated catalyst.

lected from Kamagaya City of Japan were thermally graded to obtain derived oil The composition of the municipal waste plastics from KamagayaCity is shown inTable 4.The thermal degradation was carried out at 410°C in

MWP-a continuous-flow stirred-tMWP-ank reMWP-actor The oil obtMWP-ained from municipMWP-al wMWP-asteplastics contained about 600 ppm organic chlorine compounds The physico-chemical characteristics of the derived oil were estimated using available standardmethods; the results are summarized inTable 5

The catalysts iron oxide (TR99701) and iron oxide–carbon composite(TR99300) were obtained from Toda Kogyo Corporation, Japan The Red mud,

FIG 5 Activity patterns during the sequential regeneration of the TR97305 catalyst ing the dechlorination of MX/PVC-derived oil in the absence of carrier gas

dur-Catalytic Dehalogenation of Plastic-Derived Oil 339

FIG 6 Conversion data during the dechlorination of MX/PVC-derived oil over variousiron oxide catalysts in the presence of He carrier gas

a waste byproduct of alumina production was provided by Seydisehir AluminaPlant, Turkey The ZnO and MgO were prepared by precipitation of their corre-sponding nitrates with ammonia solution The physical characteristics of the cata-lysts used in this study are shown in Table 6.The catalyst TR99701 containsmainly theγ-Fe2O3 phase The dehydrochlorination (DHC) reaction of MWP-derived oil and model chloro alkanes were carried out using a microreactor atatmospheric pressure The details of the experimental procedure have been given

in the previous section In the use of dehydrochlorination of model compounds,about 0.5 mL catalyst was loaded in the reactor and pretreated in He flow (60mL/min) at 300°C for 1 h The reactant was fed into the reactor at a flow rate

of 1.2 mL/min along with He carrier gas (30 mL/min)

FIG 7 Effect of space velocity during the dechlorination of MX/PVC-derived oil overTR97305 and TR99701 catalysts (■) γ-Fe2O3; (䊉) TR97305

FIG 8 Time-on-stream analysis: Dechlorination of MX/PVC-derived oil over differentiron oxide–carbon composite catalysts.

The distribution of all the carbon compounds (C-NP gram) in MWP oil andthe carbon number distribution of chloro-organic compounds (Cl-NP gram) arepresented in Figures 9a and 9b, respectively The C-NP gram shows that thehydrocarbons in the MWP-derived oil were distributed mainly in the b.p range

of nC7 to nC11 The Cl compounds are also distributed in almost the same range.

All the characteristics of the MWP-derived oil indicate that it can be used as afuel oil or upgraded to gasoline if the chloro-organic compounds are removed.The steady-state activity patterns of various catalysts in the dehydrochlorination

of chloro-organic compounds of MWP-derived oil at the reaction temperature of

350°C are presented in Figure 10 All the catalysts except MgO showed highactivity in the DHC of chloro-organic compounds The TR99701 catalyst, whichconsists of γ-Fe2O3, is most active in this reaction It is able to remove about95% of chloro-organic compounds from the MWP-derived oil

To understand more about the DHC activity of these catalysts, a model DHCreaction was carried using chlorocyclohexane (CCH) as a model compound Theresults of the DHC reaction of CCH over these catalysts are shown inFigure 11.The TR99701, TR99300, and Red mud showed almost stable activity withreaction time, whereas the ZnO and MgO catalysts’ activity decreased drasti-cally It is reported that in the DHC reaction, the catalytic activity decreased due

to the presence of HCl produced during the reaction [38] The XRD of the usedZnO catalyst show patterns related to ZnCl2and ZnO It suggests that part of theZnO converted to ZnCl2by reacting with HCl In the case of MgO catalyst it isdifficult to find the MgCl2phase due to its low stability at reaction temperature.However, we observed the leaching of MgO catalyst from the reactor, suggestingthat a part of MgO is converted to its chloride phase The decreased activity

in the case of ZnO and MgO catalysts might be due to the formation of theircorresponding chloride phases These oxides showed high activity at initial