- Email: [email protected]
High-temperature liquefaction of waste plastics
PII: SOO16-2361(97)00193-2
ELSEVIER
High-temperature waste plastics
Fuel Vol. 77, No. 4, pp. 293-299, 1998 0 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0016-2361/98 $19.00+0.00
liquefaction of
Prakash K. Ramdoss and Arthur R. Tarrer* Department of Chemical Engineering, Auburn University, (Received 14 March 1997; revised 15 July 1997)
Auburn
AL 36849-5128,
USA
High-temperature liquefaction of commingled post-consumer plastics obtained from the American Plastics Council was studied. The liquefaction reaction was carried out in a tubing bomb micro-reactor with operating temperatures of -5OO”C, hydrogen pressures of -790 kPa cold and reaction times of O-30 min. These high temperatures were used to reduce the residence time required for liquefaction. Reactions were carried out in the absence of catalysts. Total conversion and conversions to asphaltenes, oil, gas and coke were monitored. Total conversion as high as 100% was achieved. Gas yields up to 70% and oil yields as high as 60% were obtained. The kinetics of liquefaction of these waste materials was studied. Maximum yield of liquid product was obtained at -500°C and -5-10 min reaction time. 0 1998 Elsevier Science Ltd. All rights reserved. (Keywords: waste plastics; liquefaction; reaction kinetics)
Currently, the USA generates -20 million tonnes of plastic waste materials, 280 million automotive tires and 66 million tonnes of waste paper each year. All these wastes are discarded and end up in sanitary landfills. With existing recycling efforts, only 4% of the waste plastics are reused. Waste plastics occupy -21 vol.% of US landfills’. Increasing the recycling rate for plastics will require innovative and cost-effective recycling technologies. Recycling plastics back to their fundamental feedstocks has been one area of active research and shows promise in overcoming many of the problems that plague conventional recycling processes. These new technologies have been called ‘feedstock recycling’ or ‘advanced recycling technologies’ and include processes such as methanolysis of polyesters and thermal depolymerization of polyolefins. Advanced recycling technologies that recycle mixed plastics back to liquid petroleum feedstocks are being evaluated worldwide to understand better their technical feasibility, process economics and logistical viability. Plastics waste recycling can be categorized into four mode?. Primary recycling deals with conversion into products similar in nature to the original product. Secondary recycling involves conversion into products of different forms for less demanding applications. Tertiary recycling converts wastes into basic chemicals or feedstocks. Quaternary recycling retrieves energy from wastes through combustion. An example of the last type is incineration of wastes for power generation. Secondary recycling, which involves grinding, remelting, and re-forming of the waste materials into lower-value products such as fillers and fibres, has been a more common practice for recycling plastic wastes until now. In the USA, only 2% of the plastics waste is handled by this method. Nevertheless, plastics wastes after a number of primary and secondary recycling steps * Author to
whom correspondence should be addressed
have to be treated in the tertiary of quaternary mode. Due to strong opposition from the public regarding the incineration of waste materials, this method can no longer be an important mode of waste recycling. As a consequence, the tertiary mode of recycling of plastics wastes is gaining momentum as an alternative method. Also, the petroleum and petroleum chemical companies have started to realize that this technology can be integrated into their daily operations. Moreover, if a landfill tax of some $30-60 per tonne, which is the cost of landfilling of these materials at present, can be charged, then the process becomes profitable. This has been proved by various pilot and demonstration plants processing various types of plastic wastes in Germany, Japan, the USA and elsewhere’. Companies such as Amoco, Shell, BP, Chevron, Esso, Veba, RWE and Fuji Recycle have R&D programmes to evaluate this approach. Recently, BASF announced that it plans to build a $175 million commercial recycling plant to convert plastics wastes into a mixture of naphtha, olefins and aromatics”‘4. Most of these R&D programmes deal with hydrolysis, methanolysis and ammonolysis for condensation polymers such as poly(ethylene terephthalate) and polyurethane; hydrogenation, pyrolysis, gasification, hydrocracking, coking and visbreaking for addition polymers such as polyolefins, polystyrene and poly(viny1 chloride); and catalytic cracking. Thus, liquefaction has been proposed as a possible recycling method for plastics. Among the various methods available, high-temperature thermal liquefaction has been overlooked. In this study, high-temperature thermal liquefaction of commingled consumer plastics to produce transportation fuel was studied. The objective was to propose a reaction pathway for the formation of various products during thermal liquefaction of plastics and to develop a kinetic model for the thermal liquefaction of mixed waste plastics for the first time.
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High-temperature
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Ng et al.’ studied the pyrolysis and catalytic cracking of polythylene to transportation fuels. A high-density polyethylene was pyrolysed at 450-500°C in a closed tubing bomb reactor. It was found that the distillate yield increased at higher temperature, which also accelerated the formation of gases and coke. Secondary reactions such as saturation, isomerization, cyclization and aromatization were found to occur at high severity. In contrast, the same reaction carried out in an open system at 480°C produced less naphtha but more gas oil. The distillates contained more olefins, less saturates and very little aromatics compared with those from the closed system. Ng et al. also studied the catalytic cracking of the pyrolytic waxy products from polyethylene in a fixed-bed reactor at 5 10°C. The catalytic cracking of the pyrolytic waxy product produced high yields of gasoline and LPG of better quality than from the pyrolysis of polyethylene, with little dry gas, coke and heavy cycle oil. Songip et al.’ studied the kinetics of catalytic cracking of heavy oil obtained from waste plastics by pyrolysing polyethylene plastics at 450°C over REY zeolite catalyst. They separated the reaction products as unreacted heavy oil, gasoline, gases and coke. The rate constant obtained for each step was found to be closely proportional to the amount of strong acid sites on the catalyst. Kastner and Kaminsky4 studied the thermal cracking of polyethylene in a fixed-bed reactor over the temperature range 500-600°C. They concluded that thermal cracking of polyolefin-rich streams yielded valuable refinery and petrochemical feedstocks. At temperatures < 550°C high yields of useful products with low yields of gas and aromatics were obtained. Using recycled-product gas rather than inert gas for fluidization improved the process efficiency by increasing the aromatics production and reducing gas formation. Taghiei et aL3 studied the liquefaction of PE, PPE, PET and mixed waste plastics. They carried out the reaction at 420-45O”C, 60 min reaction time and 5.6 MPa (cold) of Hz. They used two types of catalyst: highly dispersed iron-based catalysts and an HZSM-5 zeolite catalyst. Oil yields of 8090 wt% and total conversions of 90- 100 wt% were obtained at liquefaction temperatures of 420-430°C. They concluded that an HZSM-5 catalyst is much more active at moderate temperatures (420-430°C) for the liquefaction of plastics than nanoscale, iron-based catalysts that are effective in the liquefaction of coal. They also concluded that acid cracking catalysts are very useful for direct liquefaction of plastics. In a second investigation by the same group5, it was found that low pressure (1.5 MPa, cold) of hydrogen or nitrogen gave excellent yields and that the effect of solid catalysts was smaller at higher temperature (445-460°C). A joint project was undertaken by the American Plastics Council and Conrad Industries, Inc. at Chehalis, Washington, in 1992-1994 to study the advanced recycling technology for waste plastic@. The pyrolysis was carried out in a pilot-scale reactor unit. The optimal process conditions to attain high liquid yields, good product quality, high feedstock throughput and ease of operation were determined by a parametric study. For base feedstock and plastic feedstocks containing low levels of poly(ethylene terephthalate) (PET), poly(viny1 chloride) (PVC) or intentionally added impurities, liquid yields of 65-75 wt% were achieved at pyrolysis temperatures of 480-510°C. Liquid produced from base feedstock at these temperatures contained -55 wt% aromatics and -45 wt% aliphatics. It was concluded that recycling plastics using pyrolysis is
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technically feasible and that the reactor unit was not optimized during the project. EXPERIMENTAL Materials
Commingled post-consumer plastics were obtained from the Center for Microanalysis-Waste Materials Bank, University of Utah. The material was ground, mixed uniformly and passed through a 7 10 pm screen. The plastics were mainly polyethylene and polypropylene. The proximate and ultimate analyses of the material are shown in Table 1. The material had a C:H mass ratio of 6.17. The metals analysis (by the ICP/6010 method) of the material is shown in Table 2. Except for calcium, aluminium and sodium, other metals were present in concentrations < 100 ppmw. The thermogravimetric analysis (t.g.a.) of the material is shown in Figure 1, from which it can be seen that at -500°C decomposition was almost complete. This analysis was performed to find the operating temperature range for liquefaction. The material was insoluble in hexane, tetrahydrofuran and decalin at room temperature. Hexane, tetrahydrofuran and decalin (all 299.9% purity) were obtained from Fisher Scientific Co. Procedure
High-temperature liquefaction reactions were carried out in a 50 mL tubing bomb micro-reactor in the absence of oxygen. Details of the reactor have been described earlier’. The reaction products were analysed for yields of gases, oil, asphaltenes and coke as well as amount of unreacted plastics. Reactions were carried out in the range 475-525°C at 790 kPa (cold) hydrogen pressure and with 6 g of plastic material. The reactor could be brought to the desired temperature from cold in -4 min after immersion in a sand bath. The internal reactor temperature was monitored with a
Table 1
Proximate and ultimate analyses (wt%) of waste plastics
Proximate analysis Fixed carbon Volatile matter Ash Moisture Ultimate analysis (ash-free basis) Carbon Hydrogen Nitrogen Sulfur Oxygen
Table 2 Aluminium Antimony Barium Calcium Chromium Cobalt Copper Iron
Lead Magnesium Manganese Phosphorus Sodium Zinc
Metals
content
0.74 98.8 0.45 0.01 84.6 13.7 0.65 0.01
1.o
(ppmw) of waste plastics 330 15 14 430 4.9 7.8 4.6 95 14 28 4.7 57 190 18
High-temperature
liquefaction
of waste plastics:
P. K. Ramdoss
and A. R. Tarrer
TGA
Figure 1
T.g.a. analysis of waste plastics
I
It has recently been reported8*9 that decalin at 140°C can extract unreacted polyethylene from unreacted polymercoal mixture. Details of the extraction procedure are available in the literature’-“. The decalin-insolubles consisted of coke formed during the reaction and the inorganic matter (IOM). The product analysis procedure is illustrated in Figure 2. In all these runs, the amounts of asphaltenes and preasphaltenes formed were negligible. Thus the liquid product, which was essentially hexanesoluble, was further separated into light oil and heavy oil by simulated distillation using gas chromatography.
Hexane extraction
Hexane insolubles
x
Unconverted po mer
RESULTS
Reaction pathway: Coke + IOM
Figure 2. Product Analysis Procedure
Figure 2
AND DISCUSSION
Decalin extraction
Product analysis procedure
thermocouple. Two important process variables in the hightemperature liquefaction of waste plastics are reaction temperature and reaction time. Liquefaction runs were made at reaction times of 5, 10, 15 and 30 min and temperatures of 475,490, 500 and 525°C. Additional runs were performed for model validation under conditions different from those used for parameter estimation. After the reaction, the tubing bomb reactor was cooled using an ice bath to minimize the loss of volatile compounds. Then the mass of gas formed was determined by discharging the gas. The product slurry was then removed from the reactor using hexane as solvent. The slurry was solvent-extracted with hexane to remove the oil formed. The hexane-insolubles were extracted with tetrahydrofuran to remove asphaltenes and preasphaltenes. The THF-insolubles were then extracted with decalin at 140°C.
The reaction pathway assumed for the liquefaction of waste plastics is shown in Figure 3, involving a combination of series and parallel reactions. The plastics (P) were assumed to form light oil (L), heavy oil (H), gas (G) and coke (C). Figures 4 and 5 show the product distributions obtained at three reaction temperatures and two reaction times. From the results, it can be seen that increasing the reaction time increases the amount of gas formed at all temperatures. However, the reaction time at which maximum conversion to light oil occurs is greater than that for heavy oil. The total amount of liquid product (heavy and light oils) decreases with increase in reaction temperature, but with increase in reaction time it increases to a maximum and then begins to decrease. There is no definite trend with changing reaction time for the formation of light and heavy oil with increase in reaction temperature. Model development Based on the assumed reaction pathway, a kinetic model was developed to understand the liquefaction mechanism needed for the design and scale-up of the process. The rate
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High-temperature
liquefaction
of waste plastics:
P. K. Ramdoss
and A. R. Tarrer
Parameter
P
The parameters to be estimated in this model include the eight rate constants (k 1, k 2~ k 39 k 4~ k 5, k 6~ k 79 k 8) for different temperatures. The parameter estimation problem was solved using differential algebraic optimization. The function minimized was the sum of the squares of the errors between the predicted and the experimental values. The objective function can be written as follows:
kg kt3 I“\ G \7 kl
k2
k3
k4
H
L
k6
k7
estimation
N
c
Figure 3
minS=
1
[Wp(Yp,j-Yr,i)2+Wu(Pu,i-Yu,i)2
i=l
Reaction mechanism for plastics liquefaction
+
wL(yL,
+
W&,i
i -
-
yL, iJ2 +
WG(PG,
i -
yG, ij2
Yc,J21
where wi is the weighting factor for each species, Y is the experimental mass fraction of the species and Y is the
475
500
525
Temperature (Dog C)
Figure 4 Product distribution conditions: 5 min. 790 kPa
equations corresponding be written as:
from liquefaction.
Reaction Temperature (Deg C)
to various reaction
pathways
can Figure 5 Product distribution conditions: 15 min, 790 kPa
dPldt = - (k, P + k2P + k3P)
from liauefaction.
Reaction
dHldt = k,P - k4H - k6H - k7H dLJdt = k2P + k4H - k5L - k8L
dGldt = k3P + k5L + k6H dCldt = k7H + k8L
ooooo nocoo AAAAA xxxxx ??****
where P, H, L, G and C are the mass fractions of unreacted plastics, heavy oil, light oil, gas and coke respectively. The following assumptions were made in the model development: (1) all the reactions are first-order with respect to the mass fraction of the reacting species; (2) all the reactions are irreversible; (3) there are no mass transfer resistances; (4) the temperature dependence of the rate constants is described by the Arrhenius equation.Although the variation in hydrogen concentration with time was not measured in this study, it was assumed to be constant. This is a valid assumption, since it has been reported3 that in the liquefaction of plastics, hydrogen is in fact produced rather than consumed. Also from Table 1, it can be seen that the H/C mass ratio of waste plastics is around 7 (due to the fact that it is mostly straight-chain polymer) and hence the waste is hydrogen-rich.
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Model Unreocted Heovy Oil Light Oil Cos Coke
Plastic
Time (min) Figure 6
Comparison of model and experimental
conditions:
790 kPa, 475°C
data. Reaction
High-temperature
liquefaction
Table 3
of waste plastics:
P. K. Ramdoss
and A. R. Tarrer
Rate constants (min-‘) for the formation of oils, gases
and coke ooooo
Model Unreocted
*****
Coke
_
80 2 E k
Temperature Plastic
k, kz k3 k4 ks
60
PI ; .Fl
k6 k7
40
475°C
500°C
525°C
0.1720 0.1080 0.0590 0.0590
0.2390 0.1300 0.0930 0.0630
0.3250 0.1540 0.1430 0.0670
0.0130 0.0040 0.0001
0.0250 0.0070 0.000 1
0.0450 0.0110 0.0008
f
EfSect of reaction
Time (min) Figure 7 Comparison of model and experimental data. Reaction conditions: 790 kPa, 500°C
80 2 : f;
_ ooooo OCIOO~ AAAAA
Model Unreocted Light Oil Heavy Oil
****I
Coke
Plastic
60
PC
20
0
0
10
20
Time (min)
30
Figure 8 Comparison of model and experimental data. Reaction conditions: 790 kPa, 525°C predicted mass fraction of the species, subject to the kinetic and non-negative rate constant constraints. A value of unity was assumed for all the wi’s. More details of the parameter estimation technique are discussed in earlier work”. Some of the experimental results compared with the model predictions are shown in Figures 6-8. From the graphs, it is seen that very high conversion can be obtained with high-temperature liquefaction. Conversions as high as 99 wt% were obtained at high temperature and at longer reaction times. Also, oil yields of > 60 wt% are obtained. Conversion to gas is as high as 70 wt% at higher temperatures and longer reaction times. The amount of coke formed increases with reaction time and reaction temperature, but is negligible except at higher temperatures. The effects of two important process variables-reaction time and reaction temperature-on product distribution are discussed below.
time
Liquefaction reactions were carried out at different reaction times to evaluate the effect of reaction time on the formation of various liquefaction products and to find the optimum reaction time required for the process. As the reaction time increases, more of the waste plastics is cracked to produce oil and gaseous products. Also, the heavy oil formed is cracked to form light oil and gas. Thus the heavy oil mass fraction reaches a maximum and then decreases as the reaction time increases. The reaction time at maximum heavy oil mass fraction decreases with increasing temperature. The light oil, formed directly from the plastics as well as from the heavy oil, subsequently undergoes further cracking to form gas and coke. Here again, the mass fraction of light oil formed reaches a maximum and then decreases as the reaction time increases. Increasing reaction time increases the formation of coke in the system. About 8 wt% coke is formed at a reaction time of -30 min at 525°C. The kinetic rate constants for various reaction pathways are shown in Table 3. From the results, it can be seen that increasing the reaction time increases the total conversion of plastics to various products. Conversion to gas increases almost linearly with increasing reaction time at all temperatures studied. From the kinetic rate constants for the formation of gas from light and heavy oils, it can be concluded that the rate of formation of gas from light oil is higher than that from heavy oil. This is in agreement with the results of Songip et al.’ that light oil undergoes severe cracking compared with heavy oil (obtained from plastics) to gases and gasoline. The rate of formation of coke from light oil is one order of magnitude higher than that from heavy oil. The rate constant for formation of gas from plastics is three times that from light oil and more than an order of magnitude higher than that from heavy oil. From this it can be inferred that cracking of polymer occurs to a greater extent near the end than at the centre of the polymer chain. Effect of reaction
temperature
To assess the effect of reaction temperature on conversion and formation of gases and oils, experiments were conducted at different reaction temperatures from 475 to 525°C. Conversions as high as 99.9 wt% are obtained at higher temperatures. The conversion increases from 79 to 99.5 wt% as the temperature increases from 475 to 525°C. The formation of light oil is as high as 48 wt% at 475°C and then decreases to 19 wt% as the temperature increases to 525°C. The amount of gas formed increases from 32 wt% at 475°C to 66 wt% at 525°C. The formation of coke increases from 4 wt% at 475°C to 8 wt% at 525°C. The total liquid yield decreases from 65 wt% at 475°C to 35 wt% at 525°C.
Fuel 1998 Volume
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4
297
High-temperature
liquefaction
of waste plastics:
P. K. Ramdoss
and A. R. Tarrer
80
Model oaaoo Unreacted Plastic
2
A #
0" $ 60
-6.0
PC
1
Reaction Reaction Reaction
Path Path Path
2 3
Reaction Reaction Reaction Reaction
Path Path Path Path
45\ 6 7 8
5 40 .r(
.
ic
??
0
Time Figure 9
Table 4
Model validation. Comparisonof model and experimental data at 5 10°C.Reactionconditions:790 kPa, 4OO”C, 60 min
Figure 10
Arrhenius plot of rate parameters
Kinetic parameters
Reaction path
of plastics liquefaction
Pre-exponential factor (min-‘)
Activation energy (kJ mol-‘)
4612 30 70192 0.4 4.8e6 3.5e.5 le5 1803
62.9 34.1 86.3 11.8 121.7 113.5 42.6 81.1
The reaction time at which maximum conversion to oils occurs decreases with increasing temperature. From the kinetic rate constants it can be concluded that the rate of formation of heavy oil is twice that of light oil at 525°C and decreases slightly with increase in temperature. Assuming the Arrhenius form of dependence of rate constant on temperature, the kinetic parameters of activation energy and pre-exponential factor were determined. Arrhenius plots for the various reaction paths are shown in Figure 9. The activation energies and pre-exponential factors for these pathways are listed in Table 4. From the table, it can be concluded that the selectivity for the formation of gas from plastics increases from 17 to 23% as the temperature increases from 475 to 525°C. The selectivity for the formation of light oil from plastics decreases from 32 to 24% as the temperature increases over the same range. The activation energy for the formation of gas from plastics (path 3) is 86 kJ mol-‘, which may be compared with values available in the literature: 58.6 kJ mol-’ (using CaX catalyst)” and 61.5 kJ mol-’ (using silica alumina catalyst) l3 obtained from the gasification of plastics waste. The activation for the formation of gasoline from gas oil (catalytic cracking) is -75.5 kJ mol-’ 14,compared with the value of 130 kJ mol-’ obtained in this study. The activation energy obtained in this study is slightly higher (as expected) than the data available in the literature, since the latter are for catalytic cracking process. Also, the activation energies obtained for various reaction pathways in this study are comparable with the values
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Fuel 1998 Volume
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reported by Songip et al.=. From the results, it can be concluded that as the temperature increases, conversion of plastics increases but at the same time the selectivity to liquid products decreases. Hence there exist an optimum reaction temperature and reaction time at which the liquid product yield is maximized. This optimum occurs around 500°C and 5 min residence time for the commingled postconsumer plastics used in this study. Model validation
To validate the model, liquefaction was carried out at temperatures different from those used for parameter estimation. The results are shown in Figure 10. From the plot, it can be seen that the model can predict the experimental results well within 2 3%. Thus the model is useful in extrapolating to conditions other than those used for the parameter estimation. Using this model, the optimum reaction conditions such as temperature and residence time needed for the liquefaction process to obtain maximum oil yield can be established. CONCLUSIONS In the high-temperature liquefaction of post-consumer commingled plastics, the formation of gas and coke increases with increase in either reaction time or reaction temperature. The heavy and light oils formed undergo further cracking to gas and coke. Optimum reaction conditions exist for maximizing the formation of heavy oil and light oil. A kinetic model for the thermal liquefaction of commingled plastics has been developed. The kinetic parameters of the model were evaluated using a differential algebraic optimization program. The model predictions are within + 3% of the experimental values. The activation energies and pre-exponential factors for the various reactions involved were estimated. The increased selectivity for the formation of gas at higher temperatures can be explained by the kinetic rate parameters obtained. From the results, it can be shown that a very short residence time (510 min) is required at high temperature (500°C) for
High-temperature
liquefaction
maximizing the liquid yield. Thus, thermal liquefaction of commingled plastic waste at high temperature can be used as the first stage for converting post-consumer plastics wastes into clean transportation fuel. ACKNOWLEDGEMENTS This work was supported by the US DOE under contract no. DE-FC22-93PC93053 as part of the Consortium for Fossil Fuel Liquefaction Science.
8 9 IO
REFERENCES 1 2 3
Ng, S. H., Seoud, H., Stanciulescu, M. and Sugimoto, Y., Energy & Fuels, 1995, 9, 735. Songip, A. R., Masuda, T., Kuwahara, H. and Hashimoto, K., Energy & Fuels, 1994,8, 131. Taghiei, M. M., Feng, Z., Huggins, F. E. and Huffman, G. P., Energy & Fuels, 1994, 8, 1228.
11 12 13 14
of waste plastics:
P. K. Ramdoss
and A. R. Tarrer
Kastner, H. and Kaminsky, W., Hydrocarbon Processing, 1995, 74(5), 109. Feng, Z., Zhao, J., Rockwell, J., Bailey, D. and Huffman, G., Fuel Processing Technology, 1996, 49, 17. Advanced Recycling of Plastics, Final Report with the American Plastics Council. 1995. Ramdoss, P. K. and Tarrer, A. R., Energy & Fuels, 1996,10, 996. Robbins, G. A., Winschel, R. A. and Burke, F. P., ACS Preprint, American Chemical Society, 1996. Rotenberger, K. S., Cugini, A. V. and Thompson, R. L., ACS Preprint, American Chemical Society, 1996. Ramdoss, P. K. and Tarrer, A. R., Paper to AIChE Annual Meeting, Chicago, 1996. Ramdoss, P. K., and Tarrer, A. R., Fuel Processing Technology, 1997, 51, 83. Ayame, A., Uemichi, Y., Yoshida, T. and Kanoh, H., .I. Jpn. Petroleum Inst., 1979, 22, 280. Uemichi, Y., Ayame, A., Yoshida, T. and Kanoh, H., J. Jpn. Petroleum Inst., 1980, 23, 35. Weekman, V. W. Jr and Nate, D. M.. AZChE Journal, 1970, 16, 397.
Fuel 1998 Volume
77 Number 4
299
ELSEVIER
High-temperature waste plastics
Fuel Vol. 77, No. 4, pp. 293-299, 1998 0 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0016-2361/98 $19.00+0.00
liquefaction of
Prakash K. Ramdoss and Arthur R. Tarrer* Department of Chemical Engineering, Auburn University, (Received 14 March 1997; revised 15 July 1997)
Auburn
AL 36849-5128,
USA
High-temperature liquefaction of commingled post-consumer plastics obtained from the American Plastics Council was studied. The liquefaction reaction was carried out in a tubing bomb micro-reactor with operating temperatures of -5OO”C, hydrogen pressures of -790 kPa cold and reaction times of O-30 min. These high temperatures were used to reduce the residence time required for liquefaction. Reactions were carried out in the absence of catalysts. Total conversion and conversions to asphaltenes, oil, gas and coke were monitored. Total conversion as high as 100% was achieved. Gas yields up to 70% and oil yields as high as 60% were obtained. The kinetics of liquefaction of these waste materials was studied. Maximum yield of liquid product was obtained at -500°C and -5-10 min reaction time. 0 1998 Elsevier Science Ltd. All rights reserved. (Keywords: waste plastics; liquefaction; reaction kinetics)
Currently, the USA generates -20 million tonnes of plastic waste materials, 280 million automotive tires and 66 million tonnes of waste paper each year. All these wastes are discarded and end up in sanitary landfills. With existing recycling efforts, only 4% of the waste plastics are reused. Waste plastics occupy -21 vol.% of US landfills’. Increasing the recycling rate for plastics will require innovative and cost-effective recycling technologies. Recycling plastics back to their fundamental feedstocks has been one area of active research and shows promise in overcoming many of the problems that plague conventional recycling processes. These new technologies have been called ‘feedstock recycling’ or ‘advanced recycling technologies’ and include processes such as methanolysis of polyesters and thermal depolymerization of polyolefins. Advanced recycling technologies that recycle mixed plastics back to liquid petroleum feedstocks are being evaluated worldwide to understand better their technical feasibility, process economics and logistical viability. Plastics waste recycling can be categorized into four mode?. Primary recycling deals with conversion into products similar in nature to the original product. Secondary recycling involves conversion into products of different forms for less demanding applications. Tertiary recycling converts wastes into basic chemicals or feedstocks. Quaternary recycling retrieves energy from wastes through combustion. An example of the last type is incineration of wastes for power generation. Secondary recycling, which involves grinding, remelting, and re-forming of the waste materials into lower-value products such as fillers and fibres, has been a more common practice for recycling plastic wastes until now. In the USA, only 2% of the plastics waste is handled by this method. Nevertheless, plastics wastes after a number of primary and secondary recycling steps * Author to
whom correspondence should be addressed
have to be treated in the tertiary of quaternary mode. Due to strong opposition from the public regarding the incineration of waste materials, this method can no longer be an important mode of waste recycling. As a consequence, the tertiary mode of recycling of plastics wastes is gaining momentum as an alternative method. Also, the petroleum and petroleum chemical companies have started to realize that this technology can be integrated into their daily operations. Moreover, if a landfill tax of some $30-60 per tonne, which is the cost of landfilling of these materials at present, can be charged, then the process becomes profitable. This has been proved by various pilot and demonstration plants processing various types of plastic wastes in Germany, Japan, the USA and elsewhere’. Companies such as Amoco, Shell, BP, Chevron, Esso, Veba, RWE and Fuji Recycle have R&D programmes to evaluate this approach. Recently, BASF announced that it plans to build a $175 million commercial recycling plant to convert plastics wastes into a mixture of naphtha, olefins and aromatics”‘4. Most of these R&D programmes deal with hydrolysis, methanolysis and ammonolysis for condensation polymers such as poly(ethylene terephthalate) and polyurethane; hydrogenation, pyrolysis, gasification, hydrocracking, coking and visbreaking for addition polymers such as polyolefins, polystyrene and poly(viny1 chloride); and catalytic cracking. Thus, liquefaction has been proposed as a possible recycling method for plastics. Among the various methods available, high-temperature thermal liquefaction has been overlooked. In this study, high-temperature thermal liquefaction of commingled consumer plastics to produce transportation fuel was studied. The objective was to propose a reaction pathway for the formation of various products during thermal liquefaction of plastics and to develop a kinetic model for the thermal liquefaction of mixed waste plastics for the first time.
Fuel 1998 Volume
77 Number
4
293
High-temperature
liquefaction of waste plastics: P. K. Ramdoss and A. R. Tarrer
Ng et al.’ studied the pyrolysis and catalytic cracking of polythylene to transportation fuels. A high-density polyethylene was pyrolysed at 450-500°C in a closed tubing bomb reactor. It was found that the distillate yield increased at higher temperature, which also accelerated the formation of gases and coke. Secondary reactions such as saturation, isomerization, cyclization and aromatization were found to occur at high severity. In contrast, the same reaction carried out in an open system at 480°C produced less naphtha but more gas oil. The distillates contained more olefins, less saturates and very little aromatics compared with those from the closed system. Ng et al. also studied the catalytic cracking of the pyrolytic waxy products from polyethylene in a fixed-bed reactor at 5 10°C. The catalytic cracking of the pyrolytic waxy product produced high yields of gasoline and LPG of better quality than from the pyrolysis of polyethylene, with little dry gas, coke and heavy cycle oil. Songip et al.’ studied the kinetics of catalytic cracking of heavy oil obtained from waste plastics by pyrolysing polyethylene plastics at 450°C over REY zeolite catalyst. They separated the reaction products as unreacted heavy oil, gasoline, gases and coke. The rate constant obtained for each step was found to be closely proportional to the amount of strong acid sites on the catalyst. Kastner and Kaminsky4 studied the thermal cracking of polyethylene in a fixed-bed reactor over the temperature range 500-600°C. They concluded that thermal cracking of polyolefin-rich streams yielded valuable refinery and petrochemical feedstocks. At temperatures < 550°C high yields of useful products with low yields of gas and aromatics were obtained. Using recycled-product gas rather than inert gas for fluidization improved the process efficiency by increasing the aromatics production and reducing gas formation. Taghiei et aL3 studied the liquefaction of PE, PPE, PET and mixed waste plastics. They carried out the reaction at 420-45O”C, 60 min reaction time and 5.6 MPa (cold) of Hz. They used two types of catalyst: highly dispersed iron-based catalysts and an HZSM-5 zeolite catalyst. Oil yields of 8090 wt% and total conversions of 90- 100 wt% were obtained at liquefaction temperatures of 420-430°C. They concluded that an HZSM-5 catalyst is much more active at moderate temperatures (420-430°C) for the liquefaction of plastics than nanoscale, iron-based catalysts that are effective in the liquefaction of coal. They also concluded that acid cracking catalysts are very useful for direct liquefaction of plastics. In a second investigation by the same group5, it was found that low pressure (1.5 MPa, cold) of hydrogen or nitrogen gave excellent yields and that the effect of solid catalysts was smaller at higher temperature (445-460°C). A joint project was undertaken by the American Plastics Council and Conrad Industries, Inc. at Chehalis, Washington, in 1992-1994 to study the advanced recycling technology for waste plastic@. The pyrolysis was carried out in a pilot-scale reactor unit. The optimal process conditions to attain high liquid yields, good product quality, high feedstock throughput and ease of operation were determined by a parametric study. For base feedstock and plastic feedstocks containing low levels of poly(ethylene terephthalate) (PET), poly(viny1 chloride) (PVC) or intentionally added impurities, liquid yields of 65-75 wt% were achieved at pyrolysis temperatures of 480-510°C. Liquid produced from base feedstock at these temperatures contained -55 wt% aromatics and -45 wt% aliphatics. It was concluded that recycling plastics using pyrolysis is
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technically feasible and that the reactor unit was not optimized during the project. EXPERIMENTAL Materials
Commingled post-consumer plastics were obtained from the Center for Microanalysis-Waste Materials Bank, University of Utah. The material was ground, mixed uniformly and passed through a 7 10 pm screen. The plastics were mainly polyethylene and polypropylene. The proximate and ultimate analyses of the material are shown in Table 1. The material had a C:H mass ratio of 6.17. The metals analysis (by the ICP/6010 method) of the material is shown in Table 2. Except for calcium, aluminium and sodium, other metals were present in concentrations < 100 ppmw. The thermogravimetric analysis (t.g.a.) of the material is shown in Figure 1, from which it can be seen that at -500°C decomposition was almost complete. This analysis was performed to find the operating temperature range for liquefaction. The material was insoluble in hexane, tetrahydrofuran and decalin at room temperature. Hexane, tetrahydrofuran and decalin (all 299.9% purity) were obtained from Fisher Scientific Co. Procedure
High-temperature liquefaction reactions were carried out in a 50 mL tubing bomb micro-reactor in the absence of oxygen. Details of the reactor have been described earlier’. The reaction products were analysed for yields of gases, oil, asphaltenes and coke as well as amount of unreacted plastics. Reactions were carried out in the range 475-525°C at 790 kPa (cold) hydrogen pressure and with 6 g of plastic material. The reactor could be brought to the desired temperature from cold in -4 min after immersion in a sand bath. The internal reactor temperature was monitored with a
Table 1
Proximate and ultimate analyses (wt%) of waste plastics
Proximate analysis Fixed carbon Volatile matter Ash Moisture Ultimate analysis (ash-free basis) Carbon Hydrogen Nitrogen Sulfur Oxygen
Table 2 Aluminium Antimony Barium Calcium Chromium Cobalt Copper Iron
Lead Magnesium Manganese Phosphorus Sodium Zinc
Metals
content
0.74 98.8 0.45 0.01 84.6 13.7 0.65 0.01
1.o
(ppmw) of waste plastics 330 15 14 430 4.9 7.8 4.6 95 14 28 4.7 57 190 18
High-temperature
liquefaction
of waste plastics:
P. K. Ramdoss
and A. R. Tarrer
TGA
Figure 1
T.g.a. analysis of waste plastics
I
It has recently been reported8*9 that decalin at 140°C can extract unreacted polyethylene from unreacted polymercoal mixture. Details of the extraction procedure are available in the literature’-“. The decalin-insolubles consisted of coke formed during the reaction and the inorganic matter (IOM). The product analysis procedure is illustrated in Figure 2. In all these runs, the amounts of asphaltenes and preasphaltenes formed were negligible. Thus the liquid product, which was essentially hexanesoluble, was further separated into light oil and heavy oil by simulated distillation using gas chromatography.
Hexane extraction
Hexane insolubles
x
Unconverted po mer
RESULTS
Reaction pathway: Coke + IOM
Figure 2. Product Analysis Procedure
Figure 2
AND DISCUSSION
Decalin extraction
Product analysis procedure
thermocouple. Two important process variables in the hightemperature liquefaction of waste plastics are reaction temperature and reaction time. Liquefaction runs were made at reaction times of 5, 10, 15 and 30 min and temperatures of 475,490, 500 and 525°C. Additional runs were performed for model validation under conditions different from those used for parameter estimation. After the reaction, the tubing bomb reactor was cooled using an ice bath to minimize the loss of volatile compounds. Then the mass of gas formed was determined by discharging the gas. The product slurry was then removed from the reactor using hexane as solvent. The slurry was solvent-extracted with hexane to remove the oil formed. The hexane-insolubles were extracted with tetrahydrofuran to remove asphaltenes and preasphaltenes. The THF-insolubles were then extracted with decalin at 140°C.
The reaction pathway assumed for the liquefaction of waste plastics is shown in Figure 3, involving a combination of series and parallel reactions. The plastics (P) were assumed to form light oil (L), heavy oil (H), gas (G) and coke (C). Figures 4 and 5 show the product distributions obtained at three reaction temperatures and two reaction times. From the results, it can be seen that increasing the reaction time increases the amount of gas formed at all temperatures. However, the reaction time at which maximum conversion to light oil occurs is greater than that for heavy oil. The total amount of liquid product (heavy and light oils) decreases with increase in reaction temperature, but with increase in reaction time it increases to a maximum and then begins to decrease. There is no definite trend with changing reaction time for the formation of light and heavy oil with increase in reaction temperature. Model development Based on the assumed reaction pathway, a kinetic model was developed to understand the liquefaction mechanism needed for the design and scale-up of the process. The rate
Fuel 1998 Volume
77 Number
4
295
High-temperature
liquefaction
of waste plastics:
P. K. Ramdoss
and A. R. Tarrer
Parameter
P
The parameters to be estimated in this model include the eight rate constants (k 1, k 2~ k 39 k 4~ k 5, k 6~ k 79 k 8) for different temperatures. The parameter estimation problem was solved using differential algebraic optimization. The function minimized was the sum of the squares of the errors between the predicted and the experimental values. The objective function can be written as follows:
kg kt3 I“\ G \7 kl
k2
k3
k4
H
L
k6
k7
estimation
N
c
Figure 3
minS=
1
[Wp(Yp,j-Yr,i)2+Wu(Pu,i-Yu,i)2
i=l
Reaction mechanism for plastics liquefaction
+
wL(yL,
+
W&,i
i -
-
yL, iJ2 +
WG(PG,
i -
yG, ij2
Yc,J21
where wi is the weighting factor for each species, Y is the experimental mass fraction of the species and Y is the
475
500
525
Temperature (Dog C)
Figure 4 Product distribution conditions: 5 min. 790 kPa
equations corresponding be written as:
from liquefaction.
Reaction Temperature (Deg C)
to various reaction
pathways
can Figure 5 Product distribution conditions: 15 min, 790 kPa
dPldt = - (k, P + k2P + k3P)
from liauefaction.
Reaction
dHldt = k,P - k4H - k6H - k7H dLJdt = k2P + k4H - k5L - k8L
dGldt = k3P + k5L + k6H dCldt = k7H + k8L
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where P, H, L, G and C are the mass fractions of unreacted plastics, heavy oil, light oil, gas and coke respectively. The following assumptions were made in the model development: (1) all the reactions are first-order with respect to the mass fraction of the reacting species; (2) all the reactions are irreversible; (3) there are no mass transfer resistances; (4) the temperature dependence of the rate constants is described by the Arrhenius equation.Although the variation in hydrogen concentration with time was not measured in this study, it was assumed to be constant. This is a valid assumption, since it has been reported3 that in the liquefaction of plastics, hydrogen is in fact produced rather than consumed. Also from Table 1, it can be seen that the H/C mass ratio of waste plastics is around 7 (due to the fact that it is mostly straight-chain polymer) and hence the waste is hydrogen-rich.
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Model Unreocted Heovy Oil Light Oil Cos Coke
Plastic
Time (min) Figure 6
Comparison of model and experimental
conditions:
790 kPa, 475°C
data. Reaction
High-temperature
liquefaction
Table 3
of waste plastics:
P. K. Ramdoss
and A. R. Tarrer
Rate constants (min-‘) for the formation of oils, gases
and coke ooooo
Model Unreocted
*****
Coke
_
80 2 E k
Temperature Plastic
k, kz k3 k4 ks
60
PI ; .Fl
k6 k7
40
475°C
500°C
525°C
0.1720 0.1080 0.0590 0.0590
0.2390 0.1300 0.0930 0.0630
0.3250 0.1540 0.1430 0.0670
0.0130 0.0040 0.0001
0.0250 0.0070 0.000 1
0.0450 0.0110 0.0008
f
EfSect of reaction
Time (min) Figure 7 Comparison of model and experimental data. Reaction conditions: 790 kPa, 500°C
80 2 : f;
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Model Unreocted Light Oil Heavy Oil
****I
Coke
Plastic
60
PC
20
0
0
10
20
Time (min)
30
Figure 8 Comparison of model and experimental data. Reaction conditions: 790 kPa, 525°C predicted mass fraction of the species, subject to the kinetic and non-negative rate constant constraints. A value of unity was assumed for all the wi’s. More details of the parameter estimation technique are discussed in earlier work”. Some of the experimental results compared with the model predictions are shown in Figures 6-8. From the graphs, it is seen that very high conversion can be obtained with high-temperature liquefaction. Conversions as high as 99 wt% were obtained at high temperature and at longer reaction times. Also, oil yields of > 60 wt% are obtained. Conversion to gas is as high as 70 wt% at higher temperatures and longer reaction times. The amount of coke formed increases with reaction time and reaction temperature, but is negligible except at higher temperatures. The effects of two important process variables-reaction time and reaction temperature-on product distribution are discussed below.
time
Liquefaction reactions were carried out at different reaction times to evaluate the effect of reaction time on the formation of various liquefaction products and to find the optimum reaction time required for the process. As the reaction time increases, more of the waste plastics is cracked to produce oil and gaseous products. Also, the heavy oil formed is cracked to form light oil and gas. Thus the heavy oil mass fraction reaches a maximum and then decreases as the reaction time increases. The reaction time at maximum heavy oil mass fraction decreases with increasing temperature. The light oil, formed directly from the plastics as well as from the heavy oil, subsequently undergoes further cracking to form gas and coke. Here again, the mass fraction of light oil formed reaches a maximum and then decreases as the reaction time increases. Increasing reaction time increases the formation of coke in the system. About 8 wt% coke is formed at a reaction time of -30 min at 525°C. The kinetic rate constants for various reaction pathways are shown in Table 3. From the results, it can be seen that increasing the reaction time increases the total conversion of plastics to various products. Conversion to gas increases almost linearly with increasing reaction time at all temperatures studied. From the kinetic rate constants for the formation of gas from light and heavy oils, it can be concluded that the rate of formation of gas from light oil is higher than that from heavy oil. This is in agreement with the results of Songip et al.’ that light oil undergoes severe cracking compared with heavy oil (obtained from plastics) to gases and gasoline. The rate of formation of coke from light oil is one order of magnitude higher than that from heavy oil. The rate constant for formation of gas from plastics is three times that from light oil and more than an order of magnitude higher than that from heavy oil. From this it can be inferred that cracking of polymer occurs to a greater extent near the end than at the centre of the polymer chain. Effect of reaction
temperature
To assess the effect of reaction temperature on conversion and formation of gases and oils, experiments were conducted at different reaction temperatures from 475 to 525°C. Conversions as high as 99.9 wt% are obtained at higher temperatures. The conversion increases from 79 to 99.5 wt% as the temperature increases from 475 to 525°C. The formation of light oil is as high as 48 wt% at 475°C and then decreases to 19 wt% as the temperature increases to 525°C. The amount of gas formed increases from 32 wt% at 475°C to 66 wt% at 525°C. The formation of coke increases from 4 wt% at 475°C to 8 wt% at 525°C. The total liquid yield decreases from 65 wt% at 475°C to 35 wt% at 525°C.
Fuel 1998 Volume
77 Number
4
297
High-temperature
liquefaction
of waste plastics:
P. K. Ramdoss
and A. R. Tarrer
80
Model oaaoo Unreacted Plastic
2
A #
0" $ 60
-6.0
PC
1
Reaction Reaction Reaction
Path Path Path
2 3
Reaction Reaction Reaction Reaction
Path Path Path Path
45\ 6 7 8
5 40 .r(
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ic
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0
Time Figure 9
Table 4
Model validation. Comparisonof model and experimental data at 5 10°C.Reactionconditions:790 kPa, 4OO”C, 60 min
Figure 10
Arrhenius plot of rate parameters
Kinetic parameters
Reaction path
of plastics liquefaction
Pre-exponential factor (min-‘)
Activation energy (kJ mol-‘)
4612 30 70192 0.4 4.8e6 3.5e.5 le5 1803
62.9 34.1 86.3 11.8 121.7 113.5 42.6 81.1
The reaction time at which maximum conversion to oils occurs decreases with increasing temperature. From the kinetic rate constants it can be concluded that the rate of formation of heavy oil is twice that of light oil at 525°C and decreases slightly with increase in temperature. Assuming the Arrhenius form of dependence of rate constant on temperature, the kinetic parameters of activation energy and pre-exponential factor were determined. Arrhenius plots for the various reaction paths are shown in Figure 9. The activation energies and pre-exponential factors for these pathways are listed in Table 4. From the table, it can be concluded that the selectivity for the formation of gas from plastics increases from 17 to 23% as the temperature increases from 475 to 525°C. The selectivity for the formation of light oil from plastics decreases from 32 to 24% as the temperature increases over the same range. The activation energy for the formation of gas from plastics (path 3) is 86 kJ mol-‘, which may be compared with values available in the literature: 58.6 kJ mol-’ (using CaX catalyst)” and 61.5 kJ mol-’ (using silica alumina catalyst) l3 obtained from the gasification of plastics waste. The activation for the formation of gasoline from gas oil (catalytic cracking) is -75.5 kJ mol-’ 14,compared with the value of 130 kJ mol-’ obtained in this study. The activation energy obtained in this study is slightly higher (as expected) than the data available in the literature, since the latter are for catalytic cracking process. Also, the activation energies obtained for various reaction pathways in this study are comparable with the values
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reported by Songip et al.=. From the results, it can be concluded that as the temperature increases, conversion of plastics increases but at the same time the selectivity to liquid products decreases. Hence there exist an optimum reaction temperature and reaction time at which the liquid product yield is maximized. This optimum occurs around 500°C and 5 min residence time for the commingled postconsumer plastics used in this study. Model validation
To validate the model, liquefaction was carried out at temperatures different from those used for parameter estimation. The results are shown in Figure 10. From the plot, it can be seen that the model can predict the experimental results well within 2 3%. Thus the model is useful in extrapolating to conditions other than those used for the parameter estimation. Using this model, the optimum reaction conditions such as temperature and residence time needed for the liquefaction process to obtain maximum oil yield can be established. CONCLUSIONS In the high-temperature liquefaction of post-consumer commingled plastics, the formation of gas and coke increases with increase in either reaction time or reaction temperature. The heavy and light oils formed undergo further cracking to gas and coke. Optimum reaction conditions exist for maximizing the formation of heavy oil and light oil. A kinetic model for the thermal liquefaction of commingled plastics has been developed. The kinetic parameters of the model were evaluated using a differential algebraic optimization program. The model predictions are within + 3% of the experimental values. The activation energies and pre-exponential factors for the various reactions involved were estimated. The increased selectivity for the formation of gas at higher temperatures can be explained by the kinetic rate parameters obtained. From the results, it can be shown that a very short residence time (510 min) is required at high temperature (500°C) for
High-temperature
liquefaction
maximizing the liquid yield. Thus, thermal liquefaction of commingled plastic waste at high temperature can be used as the first stage for converting post-consumer plastics wastes into clean transportation fuel. ACKNOWLEDGEMENTS This work was supported by the US DOE under contract no. DE-FC22-93PC93053 as part of the Consortium for Fossil Fuel Liquefaction Science.
8 9 IO
REFERENCES 1 2 3
Ng, S. H., Seoud, H., Stanciulescu, M. and Sugimoto, Y., Energy & Fuels, 1995, 9, 735. Songip, A. R., Masuda, T., Kuwahara, H. and Hashimoto, K., Energy & Fuels, 1994,8, 131. Taghiei, M. M., Feng, Z., Huggins, F. E. and Huffman, G. P., Energy & Fuels, 1994, 8, 1228.
11 12 13 14
of waste plastics:
P. K. Ramdoss
and A. R. Tarrer
Kastner, H. and Kaminsky, W., Hydrocarbon Processing, 1995, 74(5), 109. Feng, Z., Zhao, J., Rockwell, J., Bailey, D. and Huffman, G., Fuel Processing Technology, 1996, 49, 17. Advanced Recycling of Plastics, Final Report with the American Plastics Council. 1995. Ramdoss, P. K. and Tarrer, A. R., Energy & Fuels, 1996,10, 996. Robbins, G. A., Winschel, R. A. and Burke, F. P., ACS Preprint, American Chemical Society, 1996. Rotenberger, K. S., Cugini, A. V. and Thompson, R. L., ACS Preprint, American Chemical Society, 1996. Ramdoss, P. K. and Tarrer, A. R., Paper to AIChE Annual Meeting, Chicago, 1996. Ramdoss, P. K., and Tarrer, A. R., Fuel Processing Technology, 1997, 51, 83. Ayame, A., Uemichi, Y., Yoshida, T. and Kanoh, H., .I. Jpn. Petroleum Inst., 1979, 22, 280. Uemichi, Y., Ayame, A., Yoshida, T. and Kanoh, H., J. Jpn. Petroleum Inst., 1980, 23, 35. Weekman, V. W. Jr and Nate, D. M.. AZChE Journal, 1970, 16, 397.
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