Supercritical Fluid Extraction of Recycled Fibers: Removal of Dioxins

Chapter 4

Supercritical Fluid Extraction of Recycled Fibers: Removal of Dioxins, Stickies, and Inactivation of Microbes Downloaded by UNIV OF GUELPH LIBRARY on October 4, 2012 | http://pubs.acs.org Publication Date: October 1, 1997 | doi: 10.1021/bk-1997-0670.ch004

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Carol A. Blaney and Shafi U. Hossain

Kimberly-Clark Corporation, Long Range Research and Development, 1400 Holcomb Bridge Road, Roswell, GA 30076 Supercritical carbon dioxide and supercritical propane were found to be effective solvents in extracting stickies and trace chlorinated organics, including dioxins, from recycled fibers. These undesirable components are not effectively removed with current recycled fiber processing techniques. It was also found that endogenous yeast and mold spores on thefiberswere inactivated with supercritical carbon dioxide. An economic analysis was performed for a recycled paper pretreatment process which utilizes semi-batch supercritical fluid extraction. Matrix effects were ignored owing to lack of data. Results are encouraging, estimating cost ranges of 7-17 cents per pound offiberstreated -- costs well within reasonable price targets to pretreat premium paper and tissue products. Efficient management of solid waste streams, of which cellulose-based materials such as waste paper constitute a significant part (roughly 40%), represents a major technological challenge. In recent years, an impressive array of new technologies has emerged which addresses problems of recycling waste paper. Novel screening systems and sophisticated flotation techniques have emerged which have, in large measure, successfully addressed the problem of deinking printed stock. Bleaching sequences which avoid the use of chlorine or chlorine compounds, and rely upon hydrogen peroxide, dithionites, or formamidine sulfinic acid for attaining acceptable levels of brightness, are also under investigation in various research laboratories around the world. However, the technologies mentioned do not effectively address the problem of stickies, dioxins, and microbes in the waste paper feedstock. Recycled Fiber Contaminants Stickies. One of the problems in the use of recycledfibersis due to sticky contaminants consisting mostly of organic adhesives and tackifiers used in the converting process, such as styrene-butadiene rubbers, acrylates, and polyvinyl 1

Current address: N7753 Sundown Court, Sherwood, WI 54169 © 1997 American Chemical Society

In Supercritical Fluids; Abraham, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.

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Downloaded by UNIV OF GUELPH LIBRARY on October 4, 2012 | http://pubs.acs.org Publication Date: October 1, 1997 | doi: 10.1021/bk-1997-0670.ch004

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SUPERCRITICAL FLUIDS

polychlorinated dibenzo-para-dioxins (PCDDs or "dioxins")

2,3,7,8-tetrachlorodibenzo-para-dioxin (reportedly the most toxic congener) polychlorinated dibenzofurans (PCDFs, or "dibenzofurans") C l = 1-8 x

CI CI

.orto Ό

CI

2,3,7,8-tetrachlorodibenzofuran (reportedly the most toxic congener)

Fig. 1. Structure of Dioxin and Related Compounds


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Supercritical Fluid Extraction of Recycled Fibers


acetate. When waste papers containing these adhesives/tackifiers are defibered, stickies are broken down into various sizes rangingfrom0.05 to 0.5 mm. These stickies cause off-quality paper and paper machine downtime, sometimes requiring the use of environmentally hazardous solvents to clean equipment fouled by deposition of stickies on wire, felts, presses, rolls, and drying cylinders. Current methods to deal with stickies (additives, separation processes, and processing at high-temperatures, for example) are not sufficient to completely eliminate the problems associated with stickies in the feedstock. Eliminating stickies via furnish selection has proven unreliable and impossible to implement. Part of the experimental work presented in this paper addresses stickies problems via extracting recycled fibers with supercritical fluid, before subjecting the fibers to subsequent deinking and bleaching operations. Dioxins. One aspect of waste paper reuse is concerned with the probable presence of small quantities of reportedly toxic compounds such as "dioxins" (see Fig. 1) and similar chlorinated organic compounds in waste papers, which has received much attention recently in the news media, and has caused considerable public anxiety. Kraft pulps, when bleached with sequences including an elemental chlorine stage, sometimes contain small but detectable levels of dioxins and related chlorinated organic compounds which may be perceived by the consuming public as a health hazard. Bleached Kraft fibers under a variety of guises (e.g. coated paper, ledger paper, etc.) are often present in substantial quantities in waste paper purchased from commercial dealers. Clapp and Truemper published AOX levels in paper made from recycled fibers (1), indicating many recycled fiber feedstocks contain up to 1200 mg/kg (ppm) AOX, where AOX refers to the broad group of halogenated organics, of which dioxin and dibenzofurans are a subset. Current state-of-the-art recycled paper processing methods, which include novel screening systems and sophisticated flotation techniques, do not address removal of dioxins or related chlorinated organic compounds. The present work attacks the problem by demonstrating the removal of dioxinfromthe waste paper feed stock via supercritical fluid extraction before it is subjected to conventional deinking/bleaching operations. Microbes. The question of cleanliness and sanitation also arises when using postconsumer recycled paper products. It is therefore desirable to inactivate any potentially pathogenic organisms in the feedstock in the event that bleaching steps such as ozonation (which are presumed to sanitize) are not used. This work indicates that supercritical fluid treatment may provide a pathway towards inactivation of microbial pathogens. Experimental Section The experimental work done was designed to ascertain, in a definitive manner, if carbon dioxide or propane, under various conditions, are suitable solvents for the extraction of dioxins and stickiesfromsecondaryfibers,as well as whether these solvents are able to inactivate microbes. The quest for an unambiguous and


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pressure reduction valve

.

t e s t

m e t e r


flowmeter sample collector

extraction , v e s s e l

A £ Λ compressor heat

C0 supply 2

Fig. 2. Supercritical Fluid Extraction Apparatus.

Table 1. Stickies Content of Pulp Fibers before and after SC-C02 Extraction (Determined by Soxhiet Extraction) and Calculated Extraction Efficiency (% Removal).

Soxhiet Extr. Solvent:

Control (before SCFE)

"Product" %Stickies (after Removed SCFE) via SCFE

EtOH/Bz 1.3

0.47

64%

Acetone

0.86

0.32

63%

CH2CI2

1.2

0.25

79%

Hexane

1.0

0.45

55%


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BLANEY & HOSSAIN


expeditious answer to the question stressed the need for economy of effort and simplicity of execution in the design of the experimental program.


Dioxin Extraction Studies. Recycled paper was selected to represent both softwood and hardwoodfibersand included white and colored ledger grades and coated sulfate (magazine) papers. The papers were ground using a Wiley Mill to particle sizes of less than 0.5 mm, and subsequently spiked with 232 ppt (parts per trillion, e.g., grams per trillion grams) "dioxin". Supercritical carbon dioxide solvent extraction conditions were as follows: 71 degrees C and 34.5 Mpa (5000 psi). Extraction efficiencies were measured using a 3-liter batch extractor depicted in Fig. 2, and a supercritical carbon dioxide solventto-feed ratio of 105 (grams of solvent per gram of recycledfiber).State-of-the-art high resolution GC-MS was used to analyze the dioxin in the samples, and 5 replicates were performed for both the 'feed' and the extracted 'product'. Extraction efficiencies were greater than 95% (dioxin removal). Similar experiments were performed using supercritical propane at 125 degrees C and 34.5 MPa (5000 psi), with a solvent-to-feed ratio of 30. In this experiment, however, the feed sample was spiked with 232 ppt C (labeled) dioxin, and 'feed' and extracted 'product' samples were analyzed for both native (unlabeled) and spiked (labeled) dioxin. 13

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It was found that the supercritical propane removed 95% of the spiked dioxin, and only 85% of the native dioxin. This is not surprising. It suggests that the native dioxin is more sterically trapped inside thefiberinterstices, and/or that the binding energy of the older, native dioxin is stronger than for the spiked dioxin. This finding is in agreement with Steve Hawthorne of the University of North Dakota (2). Hawthorne found that spiked naphthalene in sludge was extracted faster than naphthalene in aged sludge contaminated with polyaromatic hydrocarbons. Stickies Experiments. Representative paperfiberscontaining stickies were used in these extraction experiments, and included ice cream cartons, paper pads with adhesive bindings, and book bindings. The paper was ground in a Wiley Mill to particle sizes of less than 0.5 mm in size. In a procedure similar to the experiments described above, the paper was extracted with carbon dioxide at 60 degrees C, 34.5 MPa (5000 psi), and a solvent-to-feed ratio of 30. Standard Soxhiet extractions were performed to determine percent extractables in the 'feed' and the extracted 'product'. The Soxhiet extraction solvents used were hexane, methylene chloride, acetone, and ethanol/benzene. Extraction efficiencies for stickies removal ranged from 64% to 79%, depending on the Soxhiet extraction solvent used. Detailed results for individual Soxhiet extractions, as well as calculations of extraction efficiencies, are shown in Table 1. Table 2 compares the extraction efficiencies of supercritical carbon dioxide with those of supercritical propane. Similar experiments to supercritical carbon dioxide extraction were performed with supercritical propane at 125 degrees C and 34.5 MPa (5000 psi), and a solventto-feed ratio also of 30. Stickies extraction efficiencies were 69% to 91%,


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depending on the Soxhiet extraction solvent used. Results for individual soxhiet extractions are summarized in the middle column of Table 2.


Another experiment using supercritical propane at 125 degrees C and only 8.26 MPa (1200 psi) gave stickies extraction efficiencies of 42 to 70 percent. Results are summarized in the right-hand column of Table 2. IR spectra of Soxhiet extraction residues show hexane only removed the natural wood extractives such as resins and fatty acids. All other solvents (and solvent blends) used showed styrene-butadiene, polyvinyl acetate and other representative stickies in the extract. Microbe Experiments. Similar experiments were performed extracting recycled fibers with carbon dioxide at 60 degrees C, 34.5 MPa (5000 psi), and a solvent-tofeed ratio of 30 (20 minute exposure time). Analysis of 'feed' samples and extracted 'product' samples confirmed inactivation of all endogenous yeast and mold spores, which is not surprising since C 0 is a waste product of these microbes. 2

Economics A detailed engineering cost analysis (CHEMCAD computer-aided design system) was performed on a solvent batch extraction process, comparing costs of using supercritical carbon dioxide as the solvent, supercritical carbon dioxide with an entraîner as the solvent, and supercritical propane as the solvent. Assumptions. The plant design was for a 100 Bone Dry Ton Per Day (BDTPD) recycled pulp/paper facility. It was assumed that the extractions were solubilitylimited (matrix effects were ignored due to a limited amount of experimental data from the extraction experiments). A 1% solvent loss rate per circulation was assumed, which is probably higher than an actual plant would run, and hence will predict costs on the high side. Note that operating temperatures and pressures were not yet optimized for this analysis, which also would tend to predict costs on the high end. It was assumed that the use of an ethanol co-solvent at concentrations of 5% ethanol in carbon dioxide would give an entraîner effect of 3. Whether this assumption is valid is debatable; however it is not unusual to experience entraîner effects much higher than 3 in such systems, especially if the solutes have polar groups which would interact with the polar hydroxyl group on ethanol. The cost of drying thefibersprior to the use of the carbon dioxide-plus-ethanol solvent (to prevent azeotrope formation of ethanol-water) was assumed to be relatively small. Propane was assumed to have an enhancement factor roughly 2 to 3 times higher than for C 0 at the same reduced temperature and density. Since the vapor pressure of the solute is likely to be about 20 times as high at the temperatures encountered for propane, the solubility in propane was given a multiplicative factor of 50 with respect to the C 0 estimated solubilities. 2

2

A semi-batch process was used in which the supercritical fluid was continuous, and the solids were loaded into several high tensile strength steel extraction vessels


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in parallel, processed, and then unloaded. The SCF stream was reduced in pressure in a separate blowdown vessel set at 5.2 MPa (750 psi), and the SCF recycled. A purge compresser was provided to remove residual SCF from the extraction vessels prior to opening, returning it to the main compressor at 5.2 MPa. An option was included for a granular activated carbon polishing step at the lower pressure to reduce contaminant levels in the SCF recycle stream. Results of Economic Analysis. A supercritical carbon dioxide semi-batch extraction facility, without the use of an entraîner, shows an initial capital investment of $74MM with an ongoing operating cost of $0.33/pound of bone dry fibers. Addition of a 5% ethanol entraîner to increase the solubilities by a factor of 3 requires a lower solvent-to-feed ratio, bringing costs down significantly and giving an initial capital investment of S40MM, with an operating cost of $0.17/pound of bone dry fibers. Using supercritical propane as a solvent allows the batch process to be run at much lower pressures. Since propane isflammable,provision was made for flushing the extraction vessels with nitrogen during loading and unloading to prevent the formation of any explosive mixture. The propane is then flared. Only when the mixture is beyond any possibility of explosion would the vessel be opened. Similarly on closing the vessel, the same process would be done in reverse to remove oxygen. This process had the lowest cost of the three discussed here, with an initial capital investment of $13MM, and $0.067/lb operating costs. The results of the economic study are given in Tables 3 and 4, and Figure 3 is a further condensed summary of the results in a bar chart. Summary and Conclusions SCFE was effective in removing the majority of dioxins (85-95%) as well as the majority (64-91%) of stickies. For stickies removal, SC-C0 at 34.5 MPa performed better than SC-propane at 8.25 MPa and worse than SC-propane at 34.5 MPa. Initial data show inactivation of common yeast and mold spores during supercritical C 0 extraction. Preliminary economics indicate that operating costs for a system using carbon dioxide with 5% ethanol co-solvent may cost nickels per pound of fibers treated, whereas liquid propane, pennies per pound. The associated fire hazards with the use of propane make carbon dioxide-based systems potentially more attractive. 2

2

The authors would like to stress that, while the results are promising, the work presented here represents the initial stages of a research program. With refinement of techniques and imaginative modification of the basic supercritical solvents, both the extraction efficiencies and the process economics are likely to improve. Recommendations The authors recommend the following research program as the next step in developing a viable, economical commercial process for pretreatment of recycled fiber feedstock with supercritical fluid extraction: In Supercritical Fluids; Abraham, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.


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Table 2. Comparisons of Extraction Efficiencies of Supercritical Solvents (Determined by Soxhiet Extraction of Sample Before and After SCFE).

Soxhiet Extr. Solvent:

S C - C 0 2 SC-Prop SC-Prop 34.5MPa 34.5MPa 8.26MPa 125 C 60 deg.C 125 C

EtOH/Bz 64%

69%

42%

Acetone

63%

86%

74%

CH2CI2

79%

91%

70%

Hexane

55%

98%

93%

Table 3. Calculated Costs of a 100 BDTPD Semi-Batch Extraction Facility to Pretreat Recycled Fibers via Supercritical Carbon Dioxide, with and w/o an Ethanol Co-Solvent. Operating Pressure

10.4MPa (1500psi)

13.8MPa (2000psi)

20.7MPa (3000psi)

Pure C02, capital costs

$89MM

$74MM

$83MM

Pure C02, cents per pound

37

33

37

C02+5% EtOH, capital costs $43MM

$40MM

$44MM

C02+5%EtOH, cents per pound

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BLANEY & HOSSAIN


Table 4. Calculated Costs of a 100 BDTPD Semi-Batch Extraction Facility to Pretreat Recycled Fibers via Supercritical Propane.

Operating Pressure

10.4MPa 13.8MPa 20.7MPa 1 500psi 2000psi 3000psi


$13MM Propane, 102 C , capital costs Propane, 102 C , cents per pound

6.7

$16MM 7.0 7.4

$13MM Propane, 121 C , capital costs Propane, 121 C , cents per pound

$15MM

6.4

$14MM $15MM 7.0 7.4

Fig. 3. Capital (SMM) and Operating Costs (cents/lb) of 100 BDTPD Supercritical Fluid Extraction Plant.


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Evaluate the effect of entraîneras) on enhancing removal of stickies and chlorinated organics such as dioxins. Investigate the ability of supercritical conditions to inactivate spiked pathogens such as hepatitis and strep. Obtain design data for solubilities, binding energies, and partitioning kinetics of stickies (styrene-butadiene, acrylates, and polyvinyl acetate) and dioxins (or model compounds which are less toxic but mimic dioxin in chemical nature). Reassess economics with revised data.

Kimberly-Clark Corporation holds four U.S. Patents covering this work (3-6). Acknowledgments The authors are grateful for the assistance provided by Phasex Corporation in Lawrence, Massachusetts in conducting the extraction experiments, and for Dr. Charles A. Eckert in the Dept. of Chemical Engineering at the Georgia Institute of Technology, Atlanta, Georgia, for his assistance in conducting the economic analysis. Literature Cited 1) Hawthorne, S., University of North Dakota, as reviewed in LC-GC vol. 13, no. 7, July 1995, pp 542-553 by Ronald E. Majors. 2) Clapp, R. T., Truemper, C. Α., Aziz, S., and Reschke, T., TappiJournal,vol. 79; no. 3, pp. 111-113, March 1996 3) Blaney, C. Α., and Hossain, S. U., US Patent No. 5,074,958, "Method For Removing Polychlorinated Dibenzodioxins And Polychlorinated Dibenzofurans And Stickies From Secondary Fibers Using Supercritical Propane Solvent Extraction". 4) Blaney, C. Α., and Hossain, S. U., US Patent No. 5,009,746, "Method For Removing Stickies From Secondary Fibers Using Supercritical CO Solvent Extraction". 2

5) Hossain, S. U., and Blaney, C. Α., US Patent No. 5,009,745, "Method For Removing Polychlorinated Dibenzodioxins And Polychlorinated Dibenzofurans From Secondary Fibers Using Supercritical CO2 Extraction". 6) Hossain, S. U., and Blaney, C. Α., US Patent No. 5,213,660, "Secondary Fiber Cellulose Product With Reduced Levels Of Polychlorinated Dibenzodioxins And Polychlorinated Dibenzofurans".


Supercritical Fluid Extraction of Recycled Fibers: Removal of Dioxins

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