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methyl tert-butyl ether affected regulated exhaust and evaporative emissions in the same general manner for the vehicles tested. Little or no effect on HC or NO, emission rates was observed with any of the aforementioned fuel supplements. Carbon monoxide emission rates were reduced significantly with all fuel blends on all four test vehicles. This reduction is summarized as a bar graph in Figure 5. An earlier study (Brinkman et al., 1975) on noncatalyst vehicles indicated similar trends. A comparison of the evaporative hydrocarbons for the various test fuels is summarized in Figure 6. Of the four test cars in the program, the fuel blends had the least effect on the Saab and Marquis. The Mustang and the Malibu showed increased evaporative emissions with two of the three fuel blends. No increase was observed in the unregulated emissions measured in the exhaust. Significant amounts of fuel additives (i-e., ethanol, TBA, or MTBE) in the evaporative emissions were observed on some tests. In general, the test-to-test variability of evaporative emissions is considerably greater than for exhaust emis-
sions. This scatter is due in part to the design and location of the specific HC evaporative emission control system. Use of fuels studied in this project should not interfere with meeting Federal Emissions Standards for gaseous emissions generated on the dynamometer portion of the FTP by the vehicles tested. Depending on the evaporative system of the vehicle, the ability of the test vehicles to meet the HC Evaporative Emission Standard was affected in some cases.
Literature Cited Brinkman, N. D.; GabpauJos, N. E.;Jackson, M. W. “Exhaust Emissions, Fuel Economy and Driveability of Vehicles Fueled with AlcohoKjasoline Blends”, Paper 750120, presented at SAE Engineering Congress, Detroit, Mlch., Feb 1975. Dietzmann. H. E., et al. “Analytical Procedures for Characterizlng Unregulated Pollutant Emissions from Motor Vehicles”; Envlronmental Science Research Laboratory, Office of Research and Development, U.S. Environmental Protection Agency, Research Triangle Park, N.C., Feb 1979. Fed. Regist. June 28, 1977, 42, No. 124, Part 111.
Received for review April 13, 1981 Accepted August 13,1981
Phosphoric Acid Systems. 2. Catalytic Conversion of Fermentation Ethanol to Ethylene Donald E. Pearson,’ Robert D. Tanner,’2 I. Daniel Plcclotto,2Jason S. Sawyer,‘ and James H. Cleveland, Jr.’ Departments of Chemistry and Chemical Engineering, Vanderbitt University, Nashville, Tennessee 37235
A bottleneck in the development of an economical chemicaVfermentation process for converting starch and cellulose into ethanol for a fuel is the separation of the alcohol from water. Recently, it has been demonstrated that the energy required for recovery by distillation can be reduced tenfold if the ethanol concentration in the recovered product is allowed to drop from 100% to 80% (by weight). One method proposed to complete the separation of water, expending rile energy, is to chemically react the microbhlly derived alcohol. It is suggested that a liquid phase catalyzed process be developed to convert the alcohol to an easily separated gas, such as ethylene, at temperatures ranging between 160 and 300 O C . The effect of process conditions such as temperature and agitation of the mixture on the rate, the yield, the water recovery, and the number of regenerations of the proposed acid catalyst is reported.
Introduction An alternative to conventional separation processes, such as distillation (Bojnowskiand Hanks, 1979),crystallization (Heist, 1979),or extraction (Hanson, 1979) for the removal of fermentation-derivedethanol from water is to chemically react the alcohol into desirable hydrocarbon products. When the original fermentation solution is pre-concentrated to around 80% by weight alcohol, a liquid polyphosphoric acid catalyzed reaction can yield ethylene (a gas) at temperatures between 150 and 300 “ C . Use of only one fractionation stage, following the suggestion of Ladisch and Dyck (1979), minimizes both energy and capital expenditures. The ethylene is easily separable from the polyphosphoric acid-alcohol-water reaction mixture: the ethylene gas evolves directly and only requires gaseous water removal for purification. At higher temperatures, Department of Chemistry Department of Chemical Engineering 0196-4321/81/1220-0734$01.25/0
the gaseous water product is also carried with the ethylene, and almost completely removed from the ethylene by passing the mixture through room temperature water. The process development, described in this paper, is both qualitative: e.g., more vigorous mixing enhances the rate of reaction, and quantitative: the effect of temperature on the reaction rate, product mix, and yield can all be described in terms of classical kinetic models. As a gasoline substitute, “ethanol fuel production now amounts to 60 million gallons per year (4000barrels per day), and is expected to reach 300 million gallons per year (20000 barrels per day) by 1982” (DOE, 1979). “Unsubsidized, fermentation ethanol currently sells for $1.20 to $1.50 per gallon. By employing advanced technology in large-scale plants, it should be possible to produce ethanol and sell it profitably at $1.00 per gallon” (DOE, 1979). In Brazil, the alcohol is being considered as raw material for chemicals now derived from imported petroleum. These chemicals include acetaldehyde, acetic acid, buta0 1981 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 4, 1981 735
diene, ethylene, and 2-ethylhexanol. The product ethylene has the advantage that it alone is produced (O’Sullivan, 1979). At the present time, however, “even with the subsidy benefit (alcohol at 7.7 cents per pound), the per pound cost of ethanol-derived ethylene of 228 is greater (188 in Brazil) than when it is made from petrochemical feedstock at current prices” (O’Sullivan, 1979). In the US., ethylene presently sells for 148 per pound (Stinson, 1979). The present prices depend on insecure petroleum imports, both for Brazil and the United States, and are subject to change. Even without an abrupt cutoff of imported petroleum, it is predicted that there will be a shortage of ethylene in the early 1980’s (Uncertainties, 1979). Since ethylene-based plastics such as low and high density polyethylene, polyvinyl chloride, and styrenics, as well as ethylene oxide and glycol (Ethylene Glycol, 1979) depend on ethylene as a raw material, it is important to have a nonpetroleum source for this and other key chemical intermediates. “Ethylene is traditionally made by the dehydration of ethanol at about 300 “C.Precise temperature control is critical. Ethers form when the process runs too cool. Aldehydes build up if the temperature is too high. Fixed-bed or fluidized bed (heterogeneous) catalyst systems are used. Typical catalysts are alumina, silica-alumina, oxides of hafnium or zirconium, and phosphoric acid on coke. Periodic catalyst regeneration is called for to remove carbon deposits” (O’Sullivan, 1979). Compared to the traditional heterogenous catalytic process, the liquid phase polyphosphoric acid scheme discussed here seems to offer several advantages: (1)At temperatures below 300 “C, only ethylene is produced even under nonisothermal conditions. (2) Regeneration of the liquid catalyst should be considerably easier than that for a solid catalyst, since the char can be more easily removed from a liquid and the liquid can be readily taken out of the reactor for reprocessing, without the usual solid abrasion losses. Typically, two parallel reactors are required for heterogeneous (solid) catalytic processes, in order to provide for regeneration. The additional capital for a second reactor does not seem necessary for the proposed liquid phase reaction. (3) Since the reaction is partially endothermic, and the liquid phase catalyzed process can easily operate at temperatures 70 “C lower than the heterogeneous solid catalyst counterpart, both the energy expended for processing and the amount of byproduct char created should be lessened. (4) Unlike the conventional catalytic process based on petroleum feedstock, this process would be suitable for moderate-scale facilities, such as those used in developing countries. (5) Again, unlike conventional processes, this process can operate with organic impurities without fouling the catalyst. Other oxygen-bearing compounds, such as those resulting from alcohol fermentations: acetalydehyde, acetone, and higher order alcohols (Bungay et al., 1979), can be converted by the same catalyst to other hydrocarbons than ethylene. Suitably reacted, those products would be liquids which could be easily separated from the gaseous ethylene. Presently, several companies are developing processes for converting alcohols into ethylene, propylene, and C4 olefins, as well as other chemicals. BASF, for example, has proposed to take the methanol produced in coal gases and react it on a zeolite catalyst to make “dimethyl ether, which, in turn, would lead to ethylene by what amounts to a dehydration step. Propylene and C4olefins also would be formed in varying amounts depending on reaction conditions’’ (BASF, 1979; and Business, 1979). An economically attractive heterogeneously catalyzed process for converting gaseous ethanol to high purity ethylene is being
commercially tested by the Scientific Design Co. (Kochar and Marcell, 1980). It was reported that “the ethanol selectivity to by-products has been reduced from 6% to about 3%,” as compared to their earlier processes. Experimental Procedure and Preliminary Results In this section we shall outline the experimental procedure and describe the synthesis of ethylene at low to moderate temperatures starting from ethanol. Ethanol is dissolved in polyphosphoric acid (PPA) and heated. Ethylene is evolved at different rates at different temperatures. The amount of ethylene formed in the reaction approaches the theoretical quantitative yields and is collected by displacing room temperature water in a 2-L graduate cylinder. The removal of water from the alcohol constitutes the loss in weight and, hence, the lowering of the maximum yield from 100% to 61% by weight. More detailed reaction conditions have been previously reported for nonisothermal operation and hence will not be repeated here (Pearson et al., 1980). What is particularly interesting to note about phosphate catalysts is that they seem to take part in basic processes in nature. Two examples are: (1) phosphorylation of intermediate compound alcohol groups in the conversion of plant carbohydrates to unsaturated hydrocarbon latexes (Calvin, 1978);and (2) the conversion of ethylene to ethanol, a possible food energy source for the first organisms, which could have taken place before amino acids were formed in a primitive earth environment ( Gellender, 1980). After the reaction the spent PPA can be regenerated by heating it under vacuum to remove the water. Any size glass flask can be employed. Typically, a 250-mL Erlenmeyer flask is used for convenience along with a 2.5-cm magnetically controlled stirring bar. It is important to remove the water product from the PPA since water lowers the catalytic activity. When the reaction is run at temperatures above 275 “C, at atmospheric pressure, the water product is removed in the off-gas and extracted in the cold water through which the product gas is bubbled. The process has been run many times with the same batch of PPA, but without catalyst regeneration the yield drops about 2% for each pass. With further study, it is expected that the process could be made continuous by recycling the PPA and removing the residual char and water in order to maintain the high catalytic activity. Reaction temperatures above 50 “C are needed to reduce the viscosity of the PPA reaction mixture in order to lower the diffusional limitation inherent to the reaction. The water-saturated gaseous product from the reaction of ethanol with PPA contains greater than 99% ethylene, on a water-free basis. The ethanol to ethylene mechanism probably is a carbonium ion process (Levenspiel, 1972), as shown C2H50H
(H+)
CH2CH3’
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-
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Details of the similar methanol to “gasoline” liquid phase catalyzed (with PPA) process have been described (Pearson, 1974,1979; Wombles, 1975). Work with other alcohols has also been discussed (Pearson et al., 1980). Results of a typical nonisothermal reaction are pictured in Figure 1. The volume of produced gas (reported at room temperature) is plotted as a function of time along with the simultaneous recording of uncontrolled temperature. Since the gas product, except for the saturated water, is essentially ethylene, about 97% of the volume is ethylene produced at standard conditions. The amount of polyphosphoric acid catalyst needed to catalyze the reaction with and without extra starting water is estimated by the number of anhydride bonds on the
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I
b-
a e y1 b
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Figure 1. A typical nonisothermal experiment in which ethylene concentration exceeded 99% on a water-free basis. Water-free ethanol reacting with polyposphoric acid catalyst.
Figure 2. Typical “apparent” zero-order kinetic rate curve for the proposed process. Greater than 99% ethylene (on a water-free basis) is produced at 160 “C.The starting concentration of ethanol is the parameter, varying between 80% and 100% by weight.
acid. For example, the P4O10 form of polyphosphoric acid has six anhydride bonds, each of which would be expected to react with one mole of alcohol. Therefore, six ethanol molecules, each of molecular weight 46, would theroretic d y react with one mole of P4Ol0of molecular weight 284, or in a ratio of 276/284 or 0.97 g ethanol to 1 g of P4O10. Based on early experiments with 4 g of ethanol, a ratio of 0.27 g of ethanol to 1g of P4O10 was found to speed up the reaction rate. An additional amount of P4O10 to tie up the initial water (and still maintain the catalytic activity) of 0.16 of H20 to 1g of P4O10 was also required for the hydrated ethanol. It is interesting to note that HzSO4, while suggested as a catalyst in the literature for this reaction, did not dehydrate the ethanol at the same reaction conditions. Instead, it led to charring as well as SOz and COz formation.
slowed to that depicted in the middle curve shown in Figure 2. And finally, when 5.2 g of extra PPA was added (for a total of 20.2 g of PPA), the rate of reaction for the 80% ethanol case became that described by the lower curve of Figure 2. Both the stirring speed (2020 rpm) and temperature (160 “C) remained constant throughout the runs, except for the 1 to 2 min initial transient in reaching 160 “C from room temperature. What seems to be important is what controls the rate of reaction. Superficially, the rate of reaction is zero order, that is for a catalytic reaction, when (P) represents the product concentration, in moles, (S) the reactant concentration, in moles, and (CAT) the starting catalyst concentration in moles, a typical rate expression would be of the form
Process Development We have investigated the effect of changes in several independent variables on the proposed reaction scheme. These variables include the temperature, intensity of mixing, polyphosphoric acid catalyst level, and initial water concentration. Their effects on the conversion of ethanol to ethylene and the rate are examined. The pressure is one atmosphere throughout. Liquid phase catalyzed processes are not as widely used industrially as heterogeneously catalyzed (by solids) ones, because of the difficulty in separating the products from the reactant-catalyst mixture (Gates et al., 1979). This problem does not occur here since a gas, ethylene, is produced and easily separated. The liquid catalyst, PPA, used here has an advantage in that the level and “lump” or “particle” size can be easily and rapidly controlled through stirring to provide desirable product rates through rapid control. A. Effect of Water Concentration on the Rate of Reaction. Following the prescription for catalyst level given earlier: 0.27 g of ethanol to 1 g of P4O10 (PPA) for the reaction and 0.16 g of HzO to 1 g of PPA to tie up the water, 4 g of ethanol were reacted with 15.0 g of PPA for the 100% ethanol, 160 “C (isothermal) reaction. The volume of gas produced (99+% ethylene, saturated with water) is depicted on the upper curve of Figure 2. With an additional 1.3 g of PPA (16.3 g total) for the 95% ethanol (5% initial water) concentration case, the rate
When (S) is much larger than the parameter, 0, eq 1simplifies to
where y is a constant for each catalyst level (here the level is that which is available for reaction, considered constant). Integrating (2), we develop a linear (in time) expression
(P) = -ft
(3)
which approximates the data in Figure 2, since neglecting the water concentration, the volume of ethylene produced is proportional to the moles of ethylene product. The linearity, observed in Figure 2 and described by eq 3, seems to imply that the rate is diffusion controlled; that is, the initial addition of water hinders the joining of ethanol and catalyst. It seems reasonable to conclude from Figure 2, therefore, that more catalyst needs to be added to overcome the hindrance effect of the initial water than that estimated by the prescribed water to PPA ratio (since the rates in the 80% and 95% cases fall below the 100% case). The data in Figure 2 are of course descriptive of a more complex scheme than just zero order, as the deviations from linearity appear to be significant. These deviations will be
Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 4, 1981
\a
84
88
92
1
96
50
100
P E R C E N T ETHHNOL
Figure 3. The effect of initial water concentration in the ethanol reactant on the yield of ethylene produced by 1.5 h. The gas is comprised of 99% or greater ethylene, on a water-free basis. The “rate” of ethylene formed is reduced by about 38% when the initial water concentration increases from 0% to 20% by weight. T = 160
“C. explored further in the analysis of the subsequent figures. The interesting observation that an 80% ethanol (by weight) mixture can be handled almost as well as the 100% case greatly strengthens our conviction that the process is appropriate for converting fermentation-derived ethanol to ethylene. The equilibrium energy of reaction of this process at 150 O C is only 7% of the heat of combustion of ethanol. This rises to 8% at 300 “C, signifying that processing energy is apparently not too significant. Another way to view the effect of water on the rate of reaction is to compare the yield at a given time (here: 1.5 h). As shown in Figure 3, the yield is normalized, relative to the amount of dry ethylene, which would be produced as an ideal gas at 25 “C and 1 atm, starting from a 4 g mixture of ethanol and water. At 100% ethanol, for example, 2126 mL of ethylene would be produced if all of the ethanol were converted to ethylene. This high yield is practically achieved in the run pictured in Figure 1; thus it is comparable to the reported yields previously mentioned (Kochar and Marcell, 1980). Since more than half is unreacted under the conditions of the experiment at 100% initial ethanol concentration, the yield is 40%. We interpret from this graph that the temperature and other control variables need to be adjusted to bring up the yield (BO%, for example, in two subsequent figures). The 80% weight ethanol results are impressive, nevertheless, when we realize that this concentration corresponds to 61% ethanol on a molar basis. B. Effect of Mixing (Mass Transfer) on the Rate of Reaction. A zero-order reaction implies that the reactant concentration does not affect the reaction rate (Daniels and Alberty, 1975). In a catalytic reaction such as this one, the “contact area” of catalyst often controls the rate so that in a “heterogenous” liquid phase reaction, the size of the catalyst “globules” is expected to control the rate. More vigorous mixing would be expected to contribute to smaller globules and, hence, greater contact area and greater reaction rate. This contention was indeed borne out in the experiments conducted using a 2.54 cm long, 0.6 cm diameter stirring bar at varying rates, as shown in Figure 4. Much is known about correlating and modeling such “diffusional limitation” effects (under such names as, say, micromixing), as evidenced by the many
IO0 150 TrnE (tl1k.1
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Figure 4. The effect of stirring on the volume of gas produced. The gas is comprised of 99% or greater ethylene, on a water-free basis. No stirring is apparently advantageous to the rate for the first half hour, while vigorous stirring enhances the rate after that time; 80% ethanol initially at 160 “C. GOO[
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Figure 5. The effect of catalyst concentrationon the volume of gas produced. The gas is comprised of 99% of greater ethylene, on a water-basis. Low catalyst levels enhance the rate for early times (up to 20 min), medium catalyst levels enhance the rate for intermediate times (between 20 and 40 min), and high catalyst levels enhance the rate for late times (after 40 min); 160 “Cand 2020 rpm stirring speed.
publications in the chemical engineering literature (Levenspiel, 1972). A particularly intriguing tentative finding is suggested from analysis of the mixing data displayed in Figure 4. An optimal mixing-rate trajectory for maximizing the rate of reaction would be no stirring for the first half hour, followed by vigorous stirring until the end of the reaction. C. Effect of Different Initial Catalyst Levels on the Rate of Reaction. It is interesting to note the effect of lowering the initial polyphosphoric acid (PPA) catalyst level below the postulated level of 20.2 g for an 80% ethanol-20% H20 (by weight) reactant mixture. At 160 OC and a stirring speed of 2020 rpm the gas produced was always 99% or more of ethylene, neglecting the water at its saturation level. A t low catalyst levels (10.8 g of PPA) the rate of gas production, interestingly, was the highest for the first 20 min, as shown in Figure 5. At intermediate catalyst levels (15.8 g PPA initially) the rate was highest between 20 and 40 min, while at the highest level (20.2 g
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 4, 1981 1803r
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Figure 6. The effect of temperature on the volume of gas produced. The gas is comprised of 99% or greater ethylene, on a water-free basis. Moderate temperatures may enhance the reaction rate over higher temperatures during the course of the reaction (around 30 min); 80% ethanol initially at 2020 rpm stirring speed.
of PPA), the rate was highest after 40 min. The analysis of Figure 5 suggests that there is an optimal time trajectory of catalyst level. Perhaps catalyst should be added to the ethanol mixture over the entire course of the reaction, rather than just initially. The marked effect of catalyst level also suggests a complex reaction-diffusion mechanism which needs to be clarified for purposes of batch reactor design and scaleup, and the design of a continuous reactor with an appropriate catalyst regeneration scheme. D. Effect of Different Temperatures on the Rate of Reaction. The dependency of the reaction rate (below 220 "C) is described for three temperatures in Figure 6. The "isothermal" runs conducted at a stirring speed of 2020 rpm were actually conducted starting from room temperature. When the catalyst and the ethanol-water mixture were first reacted, the reaction temperature rose to about 60 "C and liberated heat. After this initial exothermic reaction, the mixture was heated with an electric heating jacket to the desired temperature and maintained within f 5 "C of that indicated. As the isothermal temperature approached 300 "C, the run time was reduced to less than 5 min with nearly 100% yields above 275 "C; however, significant char formation began. The desired temperature was typically reached in a minute or two (out of typical run times of 5 to 100 min), and the initial temperature transient barely affected the shape of the curves depicted in Figure 6. As expected, the rate increased as the temperature increased, although not as fast as a doubling in rate for each 10 "C temperature rise. Fitting straight lines through the data and origin, as in Figure 2, allowed us to estimate the "zero-order" rate constant, y,as defined in eq 2. If this rate constant is considered as k(CAT), where k is expressed as min-' and (CAT) as total moles of available catalyst, it becomes a pseudo-first-order constant, suitable for use in an Arrhenius correlation as suggested by Solomon et al. (1967). In order for the dimensions of (P) and (CAT) to be consistent with the volume of gas produced in eq. 2, these variables need to be expressed in units of moles. A rate constant of approximately 4/min (leaving the dimensions as they are since we are, in fact, using k/ko)taken from the 80% ethanol line in Figure 2 (neglectingthe water content of the gas), corresponds to In k of 1.5 at 160 "C.
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