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Ind. Eng. Chem. Res. 1996, 35, 2444-2452
Thermodynamics of Methacrylate Synthesis from Methanol and a Propionate Eric H. Shreiber,† Jacqueline R. Mullen,† Makarand R. Gogate,‡ James J. Spivey,‡ and George W. Roberts*,† Department of Chemical Engineering, North Carolina State University, P.O. Box 7905, Raleigh, North Carolina 27695-7905, and Center for Engineering and Environmental Technology, Research Triangle Institute, Research Triangle Park, North Carolina 27709-2194
Methacrylate monomers can be formed by the reaction of methanol and a propionate, e.g., propionic acid, propionic anhydride, or methyl propionate. Formaldehyde, a necessary reaction intermediate, can be produced from methanol via either dehydrogenation or partial oxidation. The equilibrium compositions for several variations of this chemistry were calculated as a function of temperature, total pressure, and inlet composition using a free-energy minimization technique. The thermodynamic analysis revealed that the methanol dehydrogenation route gave higher propionate conversion and selectivity than the partial oxidation alternative. For the dehydrogenation route, both conversion and selectivity were favored by high temperature and high methanol/propionate feed ratio. The calculations also showed that a methyl propionate feed produces the least amount of water and diethyl ketone, two byproducts which can complicate any process design for this chemistry. Introduction Over 1 × 109 lb of methacrylates, e.g., methacrylic acid and methyl methacrylate, was produced in the United States in 1988, and worldwide production was at least twice that figure (Chenier, 1992). The current long-term growth rate of this family of chemicals is about 4% annually (Chenier, 1992). Methacrylate monomers are used for a myriad of products including acrylic paints and sheet and molded products. The acetone cyanohydrin process currently is used to produce 85% of the world supply of methacrylates, and is the only process used in the United States (Chenier, 1992). This process, summarized in Figure 1, has two serious environmental disadvantages. First, over 500 × 106 lb of hydrogen cyanide was used worldwide in 1988 as a raw material. Hydrogen cyanide is extremely toxic, making it difficult to handle and introducing the possibility of an environmental incident. Second, over 1 × 109 lb of ammonium bisulfate was produced worldwide in 1988 as a byproduct. Ammonium bisulfate has essentially no commercial value, and the disposal of this waste stream is costly. Historically, disposal methods have included deep wells and ocean discharge. Recently, there has been a trend toward recycling this stream to a sulfuric acid plant (Butcher, 1993). However, this leads to increased emissions of SO2 and NOx from the plant, and to increased energy consumption. Moreover, the capital cost of a facility to convert NH4HSO4 to sulfuric acid can exceed the cost of the methacrylate plant (Butcher, 1993). From both economic and environmental perspectives, a process for producing methacrylates that does not require hydrogen cyanide or produce ammonium bisulfate (or any other waste) would be attractive. Such a process can be conceptualized as shown in Figure 2, starting from methanol and a propionate derivative, such as propionic acid or propionic anhydride. The two reactions shown in Figure 2 can, in principle, be carried out in a single reactor. * To whom correspondence should be addressed. † North Carolina State University. ‡ Research Triangle Institute.
S0888-5885(95)00713-5 CCC: $12.00
Figure 1. Acetone cyanohydrin process for methyl methacrylate.
Figure 2. Methacrylate synthesis from methanol and a propionate.
The reactions of Figure 2 have been studied individually and, to a lesser extent, in concert. For example, there have been a number of publications describing the reactions of formaldehyde, introduced into the reactor as formalin, trioxane, or methylal, with a propionate, either propionic acid or methyl propionate, to produce methacrylates (Schlaefer, 1972; Albanesi and Moggi, 1983; Daniels, 1985; Guttmann and Grasselli, 1984; Ai, 1990b-d; Bailey et al., 1992). Most of these studies have been at temperatures between 200 and 500 °C and have involved the use of base or acid-base catalysts in fixed-bed reactors. © 1996 American Chemical Society
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Formaldehyde must be produced in situ from methanol in the process of Figure 2. Commercially, formaldehyde is produced by the partial oxidation of methanol, using either an iron-molybdate catalyst at temperatures from 300 to 400 °C or a silver catalyst at temperatures from 600 to 650 °C (Gerberich and Seaman, 1992; Satterfield, 1991). The equilibrium for methanol dehydrogenation to formaldehyde and hydrogen at these temperatures is unfavorable; at 400 °C, the equilibrium constant is about 0.06. The partial oxidation route can be viewed as a combination of methanol dehydrogenation with oxidation of hydrogen to shift the dehydrogenation equilibrium to the right. In a similar fashion, the condensation of formaldehyde with a propionate, the second reaction of Figure 2, may also serve to drive the dehydrogenation equilibrium. Ai (1990a,d, 1992) studied the simultaneous partial oxidation of methanol to formaldehyde and condensation of formaldehyde with an aliphatic acid or ester. All of this research involved either acetic acid or methyl acetate. The reactions were studied over heterogeneous catalysts in fixed-bed reactors operating at temperatures between 320 and 400 °C. Formaldehyde was a significant product at all conditions, suggesting that the condensation reaction was much slower than the partial oxidation of methanol. This is a significant practical problem. For economic reasons, formaldehyde would have to be recovered and recycled. However, this is difficult because a substantial amount of water is produced in the reaction. Formaldehyde and water form an azeotrope at about 37 wt% formaldehyde. Recycling the water associated with this azeotrope would have a negative effect on the reaction kinetics. Breaking the azeotrope to permit the recycle of concentrated formaldehyde would add considerable expense to the overall process. Attempts to carry out a single-step synthesis based on methanol dehydrogenation have been limited. Several investigators have studied the reaction of methanol with methyl propionate in a single reactor (Merger and Fouquet, 1982; Daniels, 1985; Ueda et al., 1985). The reaction of methanol with propionic acid, the primary raw material considered in this study, has been investigated qualitatively by Daniels (1985). At 350 °C with an aluminosilicate-supported copper catalyst and a 3/1 feed ratio of methanol to propionic acid, Daniels stated that the reaction “produced a complex mixture of products” which included methacrylic acid, methyl methacrylate, and methyl propionate (Daniels, 1985). The purpose of this study was to investigate the multiple chemical equilibria involved in the synthesis of methacrylates from methanol or dimethyl ether (CH3OCH3) and a propionate, either propionic anhydride, propionic acid, or methyl propionate. The results presented below are intended to provide a fundamental foundation to guide an ongoing experimental program of process research on this chemistry. Procedure As discussed above, the formaldehyde in Figure 2 can be generated from methanol by either dehydrogenation or partial oxidation. These process alternatives will be referred to as “methanol dehydrogenation” and “methanol oxidation”, respectively. For each alternative, three feeds were considered, propionic anhydride (PAN), propionic acid (PA), and methyl propionate (MP). Table 1 gives some of the compounds considered in this study.
Table 1. Chemical Structures and Abbreviations of Various Compounds compound
structure
abbr
propionic anhydride
O
PAN
CH3CH2C O CH3CH2C O
propionic acid
PA
O CH3CH2C OH
methyl propionate
MP
O CH3CH2C OCH3
methacrylic acid
O CH3
C
OH
CH2
methyl methacrylate
O CH3
C
MMA
C
CH2
diethyl ketone
MA
C
CH3CH2 C
OCH3 CH2CH3
DEK
O
A review of the pertinent experimental investigations cited above shows that the following independent reactions can occur when methanol is used as a source of formaldehyde:
partial oxidation of methanol to formaldehyde CH3OH + 1/2O2 S CH2O + H2O
(1)
condensation of propionic anhydride with formaldehyde to form methacrylic acid (MA) and propionic acid
CH2O + PAN S MA + PA
(2)
esterification of methacrylic acid with methanol to form methyl methacrylate (MMA)
MA + CH3OH S MMA + H2O
(3)
hydrolysis of methyl methacrylate to form methacrylic acid
MMA + H2O S MA + CH3OH
(3a)
esterification of propionic acid with methanol to form methyl propionate
PA + CH3OH S MP + H2O
(4)
hydrolysis of methyl propionate to form propionic acid
MP + H2O S PA + CH3OH
(4a)
hydrolysis of propionic anhydride to form propionic acid
PAN + H2O S 2 PA
(5)
disproportionation of propionic acid to form diethyl ketone (DEK)
2PA S DEK + CO2 + H2O
(6)
partial oxidation of methanol to carbon monoxide
CH3OH + O2 S CO + 2H2O
(7)
oxidation of carbon monoxide to carbon dioxide
CO + 1/2O2 S CO2
(8)
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Ind. Eng. Chem. Res., Vol. 35, No. 7, 1996
Table 2. Summary of Reactions and Operating Conditions for Each Process Alternativea methanol oxidation
methanol dehydrogenation
CH3OH + 1/2O2 S CH2O + H2O CH2O + PAN S MA + PA MA + CH3OH S MMA + H2O PA + CH3OH S MP + H2O PAN + H2O S 2PA 2PA S DEK + CO2 + H2O CH3OH + O2 S CO + 2H2O CO + 1/2O2 S CO2
Propionic Anhydride Feed (1) CH3OH S CH2O + H2 (2) CH2O + PAN S MA + PA (3) MA + CH3OH S MMA + H2O (4) PA + CH3OH S MP + H2O (5) PAN + H2O S 2PA (6) 2PA S DEK + CO2 + H2O (7) CO2 + H2 S CO + H2O (8)
(9) (2) (3) (4) (5) (6) (10)
CH3OH + 1/2O2 S CH2O + H2O CH2O + PA S MA + H2O MA + CH3OH S MMA + H2O PA + CH3OH S MP + H2O 2PA S DEK + CO2 + H2O CH3OH + O2 S CO + 2H2O CO + 1/2O2 S CO2
Propionic Acid Feed (1) CH3OH S CH2O + H2 (11) CH2O + PA S MA + H2O (3) MA + CH3OH S MMA + H2O (4) PA + CH3OH S MP + H2O (6) 2PA S DEK + CO2 + H2O (7) CO2 + H2 S CO + H2O (8)
(9) (11) (3) (4) (6) (10)
CH3OH + 1/2O2 S CH2O + H2O CH2O + MP S MMA + H2O MMA + H2O S MA + CH3OH MP + H2O S PA + CH3OH 2PA S DEK + CO2 + H2O CH3OH + O2 S CO + 2H2O CO + 1/2O2 S CO2
Methyl Propionate Feed (1) CH3OH S CH2O + H2 (12) CH2O + MP S MMA + H2O (3a) MMA + H2O S MA + CH3OH (4a) MP + H2O S PA + CH3OH (6) 2PA S DEK + CO2 + H2O (7) CO2 + H2 S CO + H2O (8)
(9) (12) (3a) (4a) (6) (10)
a Temperature range: 473-773 K. Pressure range: 1-15 atma. Methanol/propionate feed ratio: 0.25-4. Oxygen/methanol feed ratio: 0.1-4. Oxygen fed as air.
dehydrogenation of methanol to formaldehyde
CH3OH S CH2O + H2
(9)
H2O + CO S CO2 + H2
(10)
water gas shift
condensation of propionic acid with formaldehyde to form methacrylic acid
CH2O + PA S MA + H2O
(11)
condensation of methyl propionate with formaldehyde to form methyl methacrylate
MP + CH2O S MMA + H2O
(12)
All of the above reactions are exothermic as written, except reactions 3a, 4a, 6, and 9. Table 2 gives the reactions involved in each of the process alternatives. It is worth noting that the esterification of reaction 3 is thermodynamically favored over that of reaction 4. At 25 °C, the equilibrium constant for reaction 3 is about 3800, while the equilibrium constant for reaction 4 is about 9. The equilibrium compositions of the reaction systems shown in Table 1 were calculated as a function of temperature, pressure, and feed composition, using the Gibbs free-energy minimization module called RGIBBS in the ASPEN PLUS computer software for chemical engineering process flow-sheet simulation. The mathematical techniques on which this module is based are discussed in Gautam and Seider (1979). Each chemical compound in Table 2 is included in the ASPEN PLUS Pure Component Data Base. This data base contains the pure component thermochemical data (i.e., standard Gibbs free energy of formation, standard enthalpy of formation, and heat capacity as a function of temperature) that is required to calculate the total Gibbs free energy of the system at a specified set of conditions.
In this study, all products were assumed to be ideal gases. This assumption was checked by comparing the calculated equilibrium partial pressure of each component using three different equations of state. Calculations were performed at 473 K and 10 atma of total pressure using the ideal gas law, the Peng-Robinson (P-R) equation of state, and the Redlich-Kwong-Soave (R-K-S) equation of state. The difference in the equilibrium product composition between the ideal gas law and the P-R and R-K-S equations of state was always below 5%. The possible formation of one or more liquid phases in the product stream was not considered in the basic calculations. This assumption was checked by comparing the calculated partial pressure of each component with the equilibrium vapor pressure of the pure component liquid. At 473 K and 10 atma of total pressure, condensation of either propionic anhydride or propionic acid can occur in some cases. At 523 K and higher temperatures, the calculated partial pressures were always less than the corresponding pure component vapor pressures. In order to focus exclusively on the effects of the process variables, the possible formation of one or more liquid phases at 473 K has not been considered in the following figures. In its most fundamental embodiment, the RGIBBS module calculates the composition of the system that has the lowest Gibbs free energy for the specified temperature, pressure, and number of moles of each compound in the feed stream. The reactions taking place need not be specified. The minimization is subject to the constraints of the carbon, hydrogen, and oxygen balances. Several complications emerged when the simulations were performed in this mode. First, the analysis predicted high conversion of propionates to DEK. Although DEK is observed experimentally (Bailey et al., 1992), it is not formed in the quantities predicted by the calculation. The effects of diethyl ketone formation are discussed in detail later in this study. Second, the
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results obtained from an “unrestricted” free-energy minimization calculation could not be interpreted by assuming that only reactions 1-12 took place. More DEK was formed than could be accounted for by the amount of propionate converted. Furthermore, fewer methanol fragments, determined by the amount of formaldehyde and methacrylates in the product stream, were formed than could be accounted for by the reacted methanol. To satisfy the propionate and methanol fragment balances, three additional reactions have to be postulated:
2CH2O + H2 + MA S 2PA
(13)
5CH3OH S DEK + H2 + 4H2O
(14)
14CH3OH + CO2 S 3DEK + 13H2O
(15)
Reactions 13 and 14 were postulated for the dehydrogenation systems; reaction 15 was postulated for the oxidation system. There is no evidence in the literature to suggest that any of these reactions are kinetically significant. Thus, the unconstrained free-energy minimization is not consistent with experimental observation. If a reaction is not kinetically significant, the freeenergy minimization calculations can be restricted to eliminate or limit the extent of such a reaction. To do this in ASPEN, the specific independent reactions of interest must be specified. The maximum number of independent reactions is equal to the number of compounds minus the number of elements in the system. The reactions shown in Table 2 were specified for each of the four cases. In addition, reactions 13-15 were specified, as appropriate, and their extents of reaction were set to zero. Reaction 6 was set to 5% conversion of PA for most of the calculations. These extents are consistent with published results (Bailey et al., 1992). Definitions. Several parameters were used to characterize the system behavior. The propionate conversion was defined as
X)
moles of propionate feed reacted moles of propionate fed
The methacrylate yield, YM, was used to define the MA and MMA derived from the propionate feed:
YM )
moles of MA + MMA formed moles of propionate reacted
The methacrylate yield was defined in terms of propionate moieties so that YM ranged from 0 to 1 for each propionate feed. In the PAN system, the esterification of PA (reaction 4), the formation of DEK (reaction 6), and the presence of PA in the product stream can cause YM to be less than 1. The hydrolysis of PAN (reaction 5) can be a major contributor to effluent PA. In the PA system, reactions 4 and 6 are the sources of yield loss. In the MP system, hydrolysis to PA (reaction 4a) and DEK formation from PA are the sources of yield loss. The extent of MA esterification was quantified by defining a MMA fraction, ZMMA:
ZMMA )
outlet moles of MMA outlet moles of MA + MMA
For the methanol oxidation systems, two other parameters were defined. The combustion yield, YC,
Figure 3. Behavior of methacrylate yield, propionate conversion, and MMA fraction as a function of temperature: methanol oxidation system, PA feed.
describes the extent of methanol oxidation to oxides of carbon:
YC ) (outlet moles of CO + CO2 - moles of CO2 produced by reaction 6)/moles of CH3OH fed The CO2 fraction, ZCO2,was defined as
ZCO2 ) (outlet moles of CO2 moles of CO2 produced by reaction 6)/ (outlet moles of CO + CO2 moles of CO2 produced by reaction 6) Results The systems with different propionate feeds all exhibited similar behavior as the system parameters were varied. Detailed results will be presented only for propionic acid feed because that system showed the greatest sensitivity to variations in operating conditions. Comparisons of propionate feeds, routes to formaldehyde (oxidation or dehydrogenation), sources of formaldehyde (methanol or dimethyl ether), and restrictions to reaction 6 will be given in summary. Methanol Oxidation. Effect of Pressure. Reactions 3, 4, and 11 are independent of the total system pressure. Reactions 1 and 7 are favored by low system pressure and reaction 8 by high system pressure. The calculations reveal that there is essentially no change in the system behavior with pressure at pressures above 10 atma. Below 10 atma, the effect of pressure is relatively small. For example, at 523 K, an O2/CH3OH feed ratio of 0.5, and a CH3OH/PA feed ratio of 1, XPA and YM increase by only a few percent (absolute) as the pressure is decreased from 10 to 1 atma, and ZMMA increases by about 10% (absolute). Therefore, the total pressure was fixed at 10 atma for all subsequent calculations on the methanol oxidation system. Effect of Temperature. Figures 3 and 4 show the behavior of the methanol oxidation system as a function
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Figure 4. Behavior of combustion yield and CO2 fraction as a function of temperature: methanol oxidation system, PA feed.
of temperature with PA feed and with feed ratios of CH3OH/PA ) 1 and O2/CH3OH ) 0.50. This O2/CH3OH ratio is stoichiometric for reaction 1 and the CH3OH/ PA ratio is stoichiometric for reaction 11, assuming that all of the O2 fed is actually consumed in reaction 1. For stoichiometric O2/CH3OH and CH3OH/PA feed ratios, the equilibrium conversion, XPA, is less than 50% and decreases slightly with increasing temperature because the equilibrium for the reaction of PA with CH3OH (reaction 4) shifts to the left. This shift also accounts for some of the increase in YM with increasing temperature. The low conversion of PA is caused primarily by the competition for methanol between the partial oxidation of methanol to formaldehyde (reaction 1), the oxidation of methanol to carbon oxides (reactions 7 and 8), and the esterification of MA (reaction 3) and PA (reaction 4). Propionic acid conversion is limited by the amount of formaldehyde produced, not by the equilibria of reactions 11 and 4. Figure 4 shows that about 25% of the methanol fed is oxidized to carbon oxides. To the extent that combustion reactions occur with a stoichiometric feed, there will not be enough formaldehyde to react with all of the PA. Figure 3 shows that esterification of MA to MMA is significant over the whole temperature range of the study and is particularly favored at low temperatures. This reaction contributes to the deficiency of formaldehyde with a stoichiometric feed. The parameters ZMMA and ZCO2 both decrease with increasing temperature, while XPA, YM , and YMMA are essentially constant. The equilibria of the oxidation reactions (1), (7), and (8) all shift to the left at higher temperatures. However, since the shift is most pronounced in reaction 8, more oxygen is available to react with methanol. This causes somewhat more formaldehyde to be produced as the temperature increases, leading to a slight increase in YM. The MMA fraction, ZMMA, decreases with increasing temperature because more methanol is consumed by reactions 1 and 7, and because the equilibrium in reaction 3 shifts to the left.
Figure 5. Behavior of methacrylate yield, propionate conversion, and MMA fraction as a function of O2/CH3OH feed ratio: methanol oxidation system, PA feed.
Figure 6. Behavior of combustion yield and CO2 fraction as a function of O2/CH3OH feed ratio: methanol oxidation system, PA feed.
Effect of Oxygen/Methanol Feed Ratio. Figures 5 and 6 show the effect of O2/CH3OH feed ratio at 523 K, 10 atma, and a CH3OH/PA feed ratio of 1. The MMA fraction and PA conversion are highest when methanol is in great excess. The methacrylate yield goes through a maximum with increasing O2/CH3OH feed ratio. At low ratios, the equilibrium of reaction 1 is favored over that of reaction 7, as seen from the fact that YC in Figure 6 is essentially zero at O2/CH3OH feed ratios below about 0.25. As the feed ratio increases from 0 to about 0.25, essentially all of the added O2 goes into reaction 1, converting more of the feed methanol to formaldehyde and decreasing the amount of MP formed in reaction 4,
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Figure 7. Behavior of methacrylate yield, propionate conversion, and MMA fraction as a function of CH3OH/PA feed ratio: methanol oxidation system, PA feed.
thereby increasing YM. Beyond an O2/CH3OH feed ratio of 0.5, YM decreases with increasing feed ratio because more methanol is combusted. From Figure 6, if the O2/CH3OH feed ratio is at or above 1.5, methanol is completely combusted and none is available for partial oxidation to formaldehyde. Therefore, both YM and ZMMA are zero and both YC and ZCO2 are unity at and above this feed ratio. Reaction 6 is the only available pathway for PA conversion above this ratio. Effect of Methanol/Propionic Acid Feed Ratio. Figures 7 and 8 show the effect of methanol/propionic acid feed ratio for T ) 523 K, P ) 10 atma, and an O2/ CH3OH feed ratio of 0.50. The PA conversion approaches unity as the CH3OH/PA feed ratio approaches 4. The MMA fraction is essentially 100% for feed ratios above 2 and is close to 100% at feed ratios below 2. The methacrylate yield has a maximum value of about 80% at a feed ratio of about 3. The slight decrease at higher feed ratios is caused by the shift of reaction 4 to the right. The combustion yield and CO2 fraction remain essentially constant with varying CH3OH/PA feed ratios when the O2/CH3OH feed ratio is constant. Methanol Dehydrogenation. Effect of Pressure. Once again, the equilibrium composition was not very sensitive to total pressure, despite the fact that methanol dehydrogenation (reaction 9) is favored by low pressure. The PA conversion is essentially constant with pressure because increased esterification of PA to MP approximately offsets decreased condensation of formaldehyde with PA. The methacrylate yield, YM, decreases slightly with pressure because less of the reacted PA goes to form MA. Therefore, the total pressure was fixed at 10 atma for all subsequent calculations on the methanol dehydrogenation system. Effect of Temperature. Figure 9 shows the effect of temperature on the methanol dehydrogenation system at a pressure of 10 atma and a CH3OH/PA feed ratio of 1. The equilibrium for the dehydrogenation of methanol to formaldehyde (reaction 9) is favored by high temperatures, whereas the equilibria for the esterification reactions, (3) and (4), are favored by low temper-
Figure 8. Behavior of combustion yield and CO2 fraction as a function of CH3OH/PA feed ratio: methanol oxidation system, PA feed.
Figure 9. Behavior of methacrylate yield, propionate conversion, and MMA fraction as a function of temperature: methanol dehydrogenation system, PA feed.
atures. Therefore, in the competition for methanol, the dehydrogenation reaction is increasingly prevalent as the temperature increases. This causes the methacrylate yield to increase and the MMA fraction to decrease with temperature. Although difficult to distinguish, the PA conversion has a flat minimum of about 0.58 at 498 K, caused by competition for PA between formaldehyde (reaction 11) and methanol (reaction 4). The minimum is created by the fact that the condensation of PA to MA (reaction
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Ind. Eng. Chem. Res., Vol. 35, No. 7, 1996 Table 3. Comparison of Propionate Feeds and Formaldehyde Sources for the Dehydrogenation Processesa propionate methanol/propionate propionate feed feed ratio conversn PA PAN MP
ZMMA W/Pr
A: Formaldehyde from Methanol 2/1 0.99 0.95 0.99 4/1 1.00 0.94 0.99 1/1 1.00 0.95 0.99
propionate dimethyl ether/ propionate feed propionate feed ratio conversn PA PAN MP
YM
YM
1.89 1.40 0.91
ZMMA W/Pr
B: Formaldehyde from Dimethyl Ether 1/1 0.98 0.93 0.98 2/1 1.00 0.94 0.99 0.5/1 1.00 0.93 1.00
0.94 0.44 0.44
a 523 K, 10 atma, formaldehyde source/propionate feed stoichiometric to form 1 mol of MMA.
Figure 10. Behavior of methacrylate yield, propionate conversion, and MMA fraction as a function of CH3OH/PA feed ratio: methanol dehydrogenation system, PA feed.
11) is favored by high temperatures, while the esterification of PA with CH3OH (reaction 4) is favored by low temperatures. These trends also contribute to the increase of YM as temperature increases. At high temperatures, formation of diethyl ketone from propionic acid (reaction 6) is the main source of methacrylate yield loss. The MMA fraction, ZMMA, decreases with temperature because less methanol is available for reaction (3). Effect of Methanol/Propionic Acid Feed Ratio. From Figure 10, the PA conversion is low at low feed ratios and increases as the ratio is increased. Complete conversion of PA is reached at feed ratios above 2. The methacrylate yield, YM, is above 80% under all conditions and increases somewhat as the feed ratio increases. The maximum possible yield, neglecting DEK formation, is reached at CH3OH/PA ) 2. Discussion of Results Comparison of Propionate Feeds. As discussed earlier, the presence of water as a product of some reactions introduces major separation issues. To quantify the influence of each propionate feed on the amount of water evolved in the process, another parameter, W/Pr, was defined.
W/Pr )
outlet moles of water moles of propionate moiety reacted
The parameter W/Pr can also be influenced by the formaldehyde source. A process using dimethyl ether (CH3OCH3) as the formaldehyde source may produce less water than a process based on methanol because dimethyl ether can react with some of the water evolved:
CH3OCH3 + H2O S 2CH3OH
(16)
Table 3 gives a comparison of propionate feeds and formaldehyde sources for the dehydrogenation alternative at 573 K, 10 atma, 5% propionate conversion to DEK, and a formaldehyde source/propionate feed ratio that is stoichiometric to produce 1 mol of methyl
methacrylate. The table shows that propionate conversion (X) and ZMMA are both essentially unity for all cases. The methacrylate yield is essentially at its maximum for all cases, as conversion to DEK is the primary source of yield loss. Table 3 shows that a propionic acid feed produces the most water. This is because there is no water-consuming reaction that is favored thermodynamically. Neither the hydrolysis of MMA to MA (reaction 3a) nor hydrolysis of MP to PA (reaction 4a) has a high equilibrium constant under these conditions. By contrast, when PAN is the propionate feed, less water is produced because the hydrolysis of PAN to PA (reaction 5) is thermodynamically favorable. For systems with methanol as the formaldehyde source, MP produces the least water of any propionate feed. With both PA and PAN, water is formed from the esterification of MA to MMA (reaction 3) and the esterification of PA to MP (reaction 4). No esterification is required for a process with MP feed; MMA is formed directly from MP via condensation with formaldehyde (reaction 12). Table 3 also shows that a process with dimethyl ether as the formaldehyde source produces less water than a process with methanol as the formaldehyde source for a given propionate feed. For systems with dimethyl ether, methyl propionate and propionic anhydride produce the same amount of water. W/Pr is significantly less for dimethyl ether feed than for methanol feed because the water formed has an additional means of consumption, i.e., hydrolysis of dimethyl ether to methanol (reaction 16). Table 4 gives a comparison of propionate feeds and formaldehyde sources for the oxidation alternative at 573 K, 10 atma, 5% propionate conversion to DEK, and an oxygen/formaldehyde source/propionate feed ratio that is stoichiometric to produce 1 mol of methyl methacrylate. For all cases, ZMMA is essentially unity. The conversion and methacrylate yield for a given propionate feed are essentially the same for methanol as for dimethyl ether. The oxidation alternative exhibits the same trend with the parameter W/Pr as the dehydrogenation alternative. Comparison of Oxidation and Dehydrogenation Processes. A comparison of Table 3 with Table 4 shows that, under identical conditions, the methacrylate yield is significantly higher for the dehydrogenation process. This has important implications, since a low yield implies either the need for a large recycle of PA and/or MP or the need to resynthesize PAN from PA
Ind. Eng. Chem. Res., Vol. 35, No. 7, 1996 2451 Table 4. Comparison of Propionate Feeds and Formaldehyde Sources for the Oxidation Processesa propionate O2/methanol/ propionate feed propionate feed ratio conversn PA PAN MP
ZMMA W/Pr
A: Formaldehyde from Methanol 0.5/2/1 0.88 0.86 0.99 1/4/1 1.00 0.78 0.99 0.5/1/1 0.94 0.86 0.99
propionate O2/dimethyl ether/ propionate feed propionate feed ratio conversn PA PAN MP
YM
YM
Table 5. Formation of Diethyl Ketone in Various Dehydrogenation Processes (523 K, 10 atma)
case 2.56 2.11 1.66
ZMMA W/Pr
B: Formaldehyde from Dimethyl Ether 0.5/1/1 0.84 0.86 0.99 1/2/1 1.00 0.76 0.99 0.5/0.5/1 0.94 0.81 0.99
1.89 1.17 1.24
a
523 K, 10 atma, oxygen/formaldehyde source/propionate feed stoichiometric to form 1 mol of MMA.
and MP. For PAN feed, the higher yield in the dehydrogenation process is a consequence of the fact that the two esterification reactions are the only source of water, whereas water also is formed by two combustion reactions in the oxidation process. Water contributes to PAN conversion via reaction 5 without producing methacrylates. For PA feed, the oxidation of the formaldehyde source to carbon oxides leaves less of that feed and less oxygen available for partial oxidation to formaldehyde. This creates a more intense competition for the formaldehyde source in the oxidation system, between partial oxidation and esterification, than in the dehydrogenation system. This increased competition leads to a lower YM in the oxidation system. For MP feed, YM is lower for the oxidation process than for the dehydrogenation process because of the total oxidation of methanol to oxides of carbon. This increased consumption of methanol helps drive the hydrolysis of MP to PA (reaction 4a). This results in consumption of MP without producing methacrylates. In the oxidation systems, the competition for the formaldehyde source between the partial oxidation to either formaldehyde or carbon monoxide (reaction 7) reduces the amount of formaldehyde that can be produced. Some methanol is lost by oxidation to carbon oxides (reactions 7 and 8). This does not occur with dehydrogenation, where methanol cannot be ‘degraded’ to less valuable carbon oxides. A potential disadvantage of the methanol dehydrogenation process is that the H2 produced might saturate the double bonds in methacrylic acid and methyl methacrylate, producing isobutyric acid and methyl isobutyrate, respectively. Experimental evidence suggests that these hydrogenation reactions do occur to some extent (Merger and Fouquet, 1982; Daniels, 1985; Ueda et al., 1985) Diethyl Ketone. Table 5 shows the extent to which diethyl ketone can potentially affect the synthesis of methyl methacrylate. For each propionate feed, the conversion to DEK and the methacrylate yield were calculated for the dehydrogenation process at 573 K, 10 atma, and a methanol/propionate feed ratio that is stoichiometric to produce 1 mol of methyl methacrylate. In one case, the conversion of propionate groups to DEK was specified at 5%. In the second, “unrestricted” case, reaction 6 was allowed to come to equilibrium. Using PA as the feed provides a direct pathway to DEK, via reaction 6. Thus, the equilibrium propionate conversion to DEK is highest with PA feed. PAN feed provides an indirect pathway to DEK that is thermo-
MP: specified at 5% MP: unspecified, stoichiometric MP: unspecified, methanol lean PAN: specified at 5% PAN: unspecified, stoichiometric PAN: unspecified, methanol lean PA: specified at 5% PA: unspecified, stoichiometric PA: unspecified, methanol lean
methanol/ propionate conversn feed ratio to DEK, % 1/1 1/1 0.5/1 4/1 4/1 2/1 2/1 2/1 1/1
5.0 16.4 32.4 5.0 19.6 54.6 5.0 22.1 56.2
YM 0.95 0.84 0.68 0.95 0.80 0.45 0.95 0.78 0.44
dynamically favorable, via reaction 6, and the equilibrium conversions are almost as high for PAN as for PA. With MP feed, the only route to DEK is an indirect one that is not thermodynamically favorable, via reaction 4a. Therefore, the equilibrium (unspecified) conversion to DEK is lower for MP than for PAN. Table 5 also shows that the unrestricted conversion to DEK is much higher at lower methanol/propionate feed ratios. Propionic acid can be consumed by three different reactions: condensation with formaldehyde to form MA (reaction 11), esterification with methanol to form MP (reaction 4), and disproportionation to form DEK (reaction 6). Under the conditions studied, competition for propionic acid occurs mainly between condensation and disproportionation. At lower methanol/ propionate feed ratios, less methanol is available to make formaldehyde; therefore, more PA is available to form DEK. From the information in Table 5, it is apparent that DEK formation can be a major source of methacrylate yield loss. Any catalyst that is used in the system must be essentially inactive for reaction 6 and related reactions. Therefore, control of DEK formation will be a critical element in the successful development of any process for methacrylate synthesis from a propionate. Formaldehyde. In all of the thermodynamic simulations for both the oxidation and dehydrogenation processes, formaldehyde was never a significant product. However, Ai (1990d,e, 1992) observed substantial amounts of formaldehyde in the product stream from oxidation systems. Methanol dehydrogenation (reaction 9) can be driven by two different mechanisms: hydrogen consumption, e.g., by oxidation to form water, or formaldehyde consumption, e.g., by reactions 2 and 11. In an actual methanol oxidation system, formaldehyde will be a significant byproduct if the formaldehyde-consuming reactions are slow relative to the rate of formaldehyde formation. Conversely, the equilibrium for methanol dehydrogenation to formaldehyde (reaction 9) is driven only by formaldehyde consumption, because very little hydrogen is consumed in the water gas shift reaction (reaction 10). At the pressures and temperatures of this study, there can be no significant formaldehyde buildup in the dehydrogenation system, even if the formaldehydeconsuming reactions are slow. In experimental studies of systems similar to the present methanol dehydrogenation system, the formation of formaldehyde as a byproduct has not been reported (Merger and Fouquet, 1982; Daniels, 1985; Ueda et al., 1985; Ai, 1990a). Conclusions The ASPEN PLUS process simulation package is a very useful tool for analyzing the equilibrium behavior
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of alternative processes for the synthesis of methacrylates from propionates and methanol. The most important conclusions resulting from this work are as follows: (1) The dehydrogenation processes have higher equilibrium methacrylate yields than the oxidation processes. (2) The equilibrium methacrylate yield increases with increasing temperature. (3) The equilibrium methacrylate yield increases with increasing formaldehyde source/propionate feed ratio. (4) The equilibrium methacrylate yield is maximized at an oxygen/methanol feed ratio of about 0.5 in the oxidation process. (5) A process with methyl propionate feed produces the least amount of water at equilibrium. (6) A process with dimethyl ether as the formaldehyde source produces less water at equilibrium than a process with methanol as the formaldehyde source, for a given propionate feed. (7) Control of the formation of diethyl ketone is a crucial element of this process development. (8) The total pressure does not have a strong effect on the equilibrium behavior of any of the systems studied. (9) At equilibrium, the concentration of formaldehyde in the product stream is never significant. Literature Cited Ai, M. Reaction of Acetic Acid with Methanol over VanadiumTitanium Binary Phosphate Catalysts in the Presence of Oxygen. Appl. Catal. 1990a, 59, 227-235. Ai, M. Reaction of Propionic Acid with Methylal over VanadiumSilicon-Phosphorous Oxide. Appl. Catal. 1990b, 63, 365-373. Ai, M. Reaction of Methyl Propionate with Methylal over V-Si-P Ternary Oxide Catalysts. Bull. Chem. Soc. Jpn. 1990c, 63, 3722-3724. Ai, M. The Effects of the Reaction Variables on the Yields of Acrylic Acid and Methyl Acrylate in the Reaction of Acetic Acid with Methanol in the Presence of Oxygen. Chem. Soc. Jpn. 1990d, 63, 199-202.
Ai, M. The Production of Methacrylic Acid by the Vapor-Phase Aldol Condensation over V-Si-P Ternary Oxide Catalyst. Chem. Soc. Jpn. 1990e, 63, 1217-1220. Ai, M. Reaction of Methyl Acetate with Methylal in the Presence of Oxygen. Stud. Surf. Sci. Catal. 1992, 72, 101-108. Albanesi, G.; Moggi, P. Methyl Methacrylate by Gas Phase Catalytic Condensation of Formaldehyde with Methyl Propionate. Appl. Catal. 1983, 6, 293-306. Bailey, O. H.; Montag, R. A.; Yoo, J. S. Methacrylic Acid Synthesis I. Condensation of Propionic Acid with Formaldehyde over Alkali Metal Cation on Silica Catalysts. Appl. Catal. A 1992, 88, 163-177. Butcher, C. Transparent ambitions. Chem. Eng. (London, England) 1993, 541, 12. Chenier, P. J. Survey of Industrial Chemistry, 2nd revised ed.; VCH, New York, 1992; p. 248. Daniels, J. A. European Patent 0138295, April 24, 1985. Gautam, R.; Seider, W. D. Computation of Phase and Chemical Equilibrium. AIChE J. 1979, 25, 991-1006. Gerberich, H. R.; Seaman, G. C. Formaldehyde. In Kirk-Othmer Encyclopedia of Chemical Technology, 4th ed.; Wiley-Interscience: New York, 1992; Vol. 11, pp 935-939. Guttmann, A. T.; Grasselli, R. K. European Patent 0124380, July 11, 1984. Merger, F.; Fouquet, G. Preparation of Methyl Methacrylate by Reaction of Methyl Propionate and Methanol. U.S. Patent 4,336,403, 1982. Satterfield, C. N Heterogeneous Catalysis in Industrial Practice, 2nd ed.; McGraw-Hill: New York, 1991; pp 285-290. Schlaefer, F. W. Production of Unsaturated Acids, Esters, and Nitriles, and Catalyst Therefor. U.S. Patent 3,933,888, 1972. Ueda, W.; Kurokawa, H.; Moro-Oka, Y.; Ikawa, T. Coupling Reaction Between Methylpropionate and Methanol to Form Methylmethacrylate over Metal Ion-Contained Magnesium Oxide Catalysts. Chem. Lett. 1985, 819-820.
Received for review November 28, 1995 Accepted April 30, 1996X IE9507134
X Abstract published in Advance ACS Abstracts, June 15, 1996.
