Ind. Eng. Chem. Res. 1994,33, 181-184
181
KINETICS, CATALYSIS, AND REACTION ENGINEERING Kinetic Study of Methylamine Reforming over a Silica-Alumina Catalyst John W. Mitchell,' Kathryn S. Hayes, and Eugene G. Lutz Air Products and Chemicals, Inc., 7201 Hamilton Boulevard, Allentown, Pennsylvania 18195-1501 The performance of a commercial silica-alumina catalyst was investigated in the methylamine reforming reaction system. A mass action based kinetic model was derived from experiments conducted in a laboratory backmixed reactor. Application of this model identified a kinetic route to dimethylamine formation that indicates that operation under kinetic control clearly is necessary t o achieve maximum dimethylamine selectivity over this catalyst.
Introduction Methylamines are prepared industrially via the exothermic reaction of methanol and ammonia over a solid acid catalyst (van Gysel and Musin, 19881, typically an amorphous silica-alumina. The kinetics of the overall synthesis reaction have been studied by a variety of investigators. Deeba et al. (1984) provide methanol conversion rates for a variety of silica-alumina catalysts, both amorphous and crystalline. The synthesis, typically carried out at 350-400 OC (Rohm 8z Haas, 1965),proceeds to a thermodynamic mix of the three amines (Leonard Process Company, 1979)monomethylamine (MMA, CH3NH2), dimethylamine (DMA, (CH&NH), and trimethylamine (TMA, (CH3)sN). The equilibrium distribution favors the formation of TMA, while the largest market is for DMA (Gibson et al., 1984). Once formed, the methylamine species may interconvert via reforming reactions. These reactions are 2MMA = DMA + NH3
+ MMA DMA + MMA = TMA + NH3 2DMA = TMA
(1) (2)
(3)
Understanding the equilibrium and kinetics of these reforming reactions is key to developing a scheme to optimize formation of the commercially desirable DMA. In current industrial practice, much of the TMA and MMA products are recycled back to the converter to produce additional DMA. If a kinetic path to DMA could be identified, TMA and/or MMA recycle could be reduced and the overall process efficiency increased. Issoire and van Long (1960) measured the chemical equilibrium constants for methylamines reforming over an alumina catalyst at various temperatures and compared their results to thermodynamic predictions. They developed the following temperature-dependent expressions to describe the reforming equilibria: log K, = (1028.4/T~)- 0.877
Table 1. Rate Constants of Reforming Reactions at 1 atm (from Weiaert. 1987) reaction no. Na-mordenitea D-9700 AL-010G.b 1 0.09 4 27 2 0.0022 7.2 0.66 3 0.045 20.4 13.5 a Values relative to the rate of methanol amination. b Relative values from MMA disproportionation.
Schmitz (1975) measured the pseudoequilibrium constants for the reforming reactions during his study on the kinetics of the competition between methanol dehydration and amination over silica-aluminas. The values that he determined at 425 OC showed good agreement with the values calculated from the equations developed by Issoire and van Long (1960). There is only one literature report of direct measurement of methylamine reforming kinetics. Restelli and Coull (1966) studied reforming of MMA and DMA over a montmorillonite clay catalyst and found that the rates of DMA reforming were comparable to the rates of MMA reforming at temperatures that were approximately 80 "C lower. The rates of MMA reforming were determined to be proportional to the partial pressure of MMA while DMA reforming is controlled by a surface reaction so that the rate is proportional to the square root of the DMA partial pressure. Weigert (1981, 1987) obtained relative rates of the reforming reactions over sodium mordenite, Davison D-970 silica-alumina, and Harshaw AL-0104 alumina through his analysis of the kinetics of the methanol amination reaction. His results, displayed in Table 1, show that the nature of the catalyst has a significant effect on the relative rates of the reforming reactions. Due to the limited information concerning the reforming reactions, we have performed a laboratory study to measure the kinetics of methylamine reforming over a commercial amorphous silica-alumina catalyst. Using this data, our goal was to develop a mathematical model describing methylamine reforming.
(4)
Experimental Section Methylamine feedstocks were obtained from either Air Products or Matheson. Ammonia was obtained from Air Products. The LA-30 catalyst, used for all of the exper0888-588519412633-0181$04.50/0 0 1994 American Chemical Society
182 Ind. Eng. Chem. Res., Vol. 33, No. 2, 1994 Table 2. Composition and Physical Properties of LA-30 composition (wt % ) A1203 13.5 Si02 86.3 NazO 0.10 0.04 so4 0.03 Fe physical properties surface area, m2/g 130 0.5 pore volume, cmYg bulk density, g/cm3 0.72
iments conducted in this study, was obtained from Akzo. Table 2 shows the composition and physical properties of LA-30. Berty Reactor. For the kinetic experiments, a Berty (an intensely backmixed reactor that has a high internal recycle ratio so that the concentrations of the various components in the reaction mixture are nearly uniform throughout the fluid) reactor was charged with 5 cm3 (4.07 g) of -20/+30 mesh LA-30 catalyst. The pressure was set to 1825kPausing a back-pressure regulator, and the reactor was heated to about 150"C under a nitrogen purge. When the reactor temperature was at least 150 "C, the flow of methylamines was initiated, the nitrogen purge was shut off, and the agitator speed was set to 1500 rpm. After the temperature stabilized, a gas chromatograph (GC) was started and data collection was begun. The effluent from the reactor was split into two portions with one portion directed through the gas sampling valves mounted on the GC and the other portion directed to a scrubber (HzS04 solution). The flow to the gas sampling valves was controlled with a flowmeter. A second back-pressure regulator set to 308-377 kPa was used to reduce the pressure of the effluent stream fed to the GC, and a portion of the line connected to the gas sampling valves was heated to 100 "C to ensure that the amines vaporized. Data taken at low space velocities (1000 and 2000 h-' (GHSV)) were collected over a period of 2-3 days. Higher space velocity data (4000-10 000 h-1 GHSV) was collected during a single day. Analysis. The gas samples were analyzed on-line using a HP 5890 GC equipped with a 3365 ChemStation. The column was a 183-cmX 0.318-cm-0.d. stainless steel column packed with 4% Carbowax 20M + 0.8% KOH on 60180 mesh Carbopack B. The injector temperature was set at 200 "C, while the detector temperature was maintained at 175 "C. Samples were analyzed every hour.
Results and Discussion Reforming Kinetics over LA-30. Commercial methanol amination reactors typically operate at around 399 "C. Initial screening studies, conducted in a plug flow reactor containing -12/+18 mesh catalyst particles, suggested that at this temperature diffusion limitations may suppress kinetic rates. Therefore, our preliminary experiments focused on evaluation of internal and external diffusion effects. Results from the plug flow screening data showed that the DMA reforming reaction is the fastest of the three reactions. Consequently, we examined the effects of internal diffusion by reacting DMA at 399 "C over LA-30 having various particle sizes. Since the feedstock is pure DMA, the reaction rate is a function of the outlet DMA concentration. Lower DMA concentrations in the reactor effluent are indicative of faster reaction rates. The results presented in Table 3 show that diffusion limitations affect the reaction rates for 0.318-cm pellets and for -12/+18 mesh particles; the rate appears to be kinetically controlled for the -20/+30 and -30/+40 par-
Table 3. Effect of Catalyst Size on DMA Reaction Rate at 3911 o
c
wt % DMA in reactor
catalyst size 0.318-cm pellet -12/+18 -20/+30 -20/+30 -30/+40 -30/+40
GHSV,h-' 8900 8900 8900 13350 8900 13350
effluent (std dev) 50.66 (0.87) 42.13 (1.00) 32.12 (0.84) 44.53 (0.54) 33.18 (1.06) 42.00 (0.67)
Table 4. Effect of Agitator Speed on DMA Reaction Rate at 399 "C agitator w t % DMA in reactor meed, rDm GHSV,h-l effluent (std dev) 400 16900 57.64 (1.04) 800 16900 49.76 (1.97) 1500 16900 52.42 (1.35)
ticles. External diffusion limitations can be identified by studying the effect of linear velocity on observed rates. This is done in a Berty reactor by varying the agitator speed at constant temperature and feed concentrations. The results of the agitator speed study presented in Table 4 show that external diffusion limitations are not a concern at agitator speeds above 800 rpm. On the basis of these results, experiments to obtain reforming kinetic data were conducted using -20/+30 mesh catalyst particles, at an agitator speed of 1500 rpm. During the diffusion study, we found that because the DMA reforming reaction is very fast at 399 "C, some DMA decomposes to heavies and coke. Decomposition of DMA in the absence of catalyst also was observed at this temperature. Therefore, although we originally planned to study the reforming reactions at temperatures of 343, 371, and 399 "C, we decreased the temperature range of the kinetic study to 300,314, and 328 "C. We also verified that DMA does not thermally decompose in the Berty reactor below 371 "C. The kinetic data were collected at the three temperatures listed above and at various space velocities using either pure DMA or pure MMA as feedstock. Space velocities were chosen so that the outlet concentration of the feed component would be above 50% for most observations at each temperature, and thus be above the equilibrium concentration. A tabulation of the kinetic data is presented as Table 5. Data for each set of conditions represent an average of several measurements. Kinetic Analysis. The data from the Berty reactor studies were analyzed to determine if a mathematical description, for use in simulation studies, could be established. The Berty reactor is a gradientless reactor; the reactions proceed at a single value of rate determined only by the outlet conditions of the reactor. A steadystate material balance across the reactor shows that the component rates of change are given by (7)
where Ri = rate of consumption of component i, g-mol/(s.g of catalyst); Qf = volumetric flow of feed to reactor, L/s; Q. = volumetric flow of product from reactor, L/s; Cif = concentration of component i in reactor feed, g-rnol/L; Ci, = concentration of component i in reactor exit, g-mol/L; and W = weight of catalyst in reactor, g. The component rates are related to the individual reaction rates through the stoichiometry as given by nr
Ri =
EAij'; j=l
where Aij = stoichiometric coefficient for component i in
Ind. Eng. Chem. Res., Vol. 33, No. 2, 1994 183 Table 5. Reforming Kinetic Data effluent comDosition. w t % T,OC GHSV,h-l NHa MMA DMA 'TMA feed 78.83 9.91 4.32 MMA 6.94 327 3000 MMA 8.31 81.07 7.58 3.05 328 4000 11.70 12.19 MMA 19.06 57.05 328 1000 1.78 5.94 MMA 5.99 86.29 328 6000 MMA 17.21 66.41 9.02 7.36 328 2000 DMA 0.63 9.82 67.72 21.83 300 2800 MMA 9.22 73.28 10.92 6.59 1000 300 DMA 0.15 5.53 83.36 10.95 300 6000 DMA 0.38 7.89 75.01 16.72 302 4000 DMA 0.10 4.70 86.06 9.15 301 8000 MMA 13.24 58.92 14.84 13.00 314 1000 DMA 1.57 12.30 54.94 31.18 316 2800 7.03 78.10 14.56 DMA 0.31 314 8000 8.50 72.20 18.79 DMA 0.51 314 6000 DMA 1.10 10.92 62.60 25.39 315 4000 MMA 9.54 72.76 11.14 6.57 300 1000 MMA 3.36 89.56 5.83 1.25 300 3000 9.73 7.97 MMA 15.07 67.22 750 300 3.58 0.50 5000 MMA 2.55 93.38 300 6.64 1.79 MMA 4.45 87.12 2000 300 MMA 11.76 75.55 8.34 4.35 2000 314 MMA 3.14 90.66 5.29 0.91 6000 315 10.31 6.05 MMA 11.37 72.27 1500 314 11.28 10.08 MMA 16.31 62.32 1000 314 DMA 2.33 12.80 47.93 36.95 2800 328 loo00 DMA 0.30 7.18 77.01 15.51 326 DMA 1.58 11.37 59.24 27.81 4500 328 67.69 22.06 0.76 9.49 6000 DMA 326 MMA 19.23 46.97 15.35 18.46 1000 328 MMA 6.12 82.87 8.17 2.84 6000 327 MMA 20.00 61.26 9.88 8.86 2000 327 MMA 15.51 68.18 9.83 6.49 3000 328
on ""
~~
'
10
Y
1
10.1 1.66
1.66
1.69
1.70
1.71
1.72
1.73
1.74
/'.
./ Parity
-
8 w
-20 -40
-
-60
-
I-
2
................
o-...
4"
w
I
20-
m
-80 -80
50 40
-$-
"3
30 v)
20 10 0
.a......................... A
2500
-
A
...................
I
I
4500
6500
GHSV, hr
8500
-'
Figure 3. Model predictions a t 300 OC;DMA feed.
and some function of composition. We tried a variety of mathematical models, but found that a simple mass action based kinetic functionality describes the system quite well. For the three reforming reactions the rate expressions are assumed to be
where the three equilibrium constants (K1, K z , K3) are taken as those reported by Issoire and van Long (1960). The kinetic constants (kl, kz, k3) were determined, via nonlinear regression, at each of the three temperatures studied. The regression was based on a modified Levenberg-Marquardt algorithm (Marquardt, 1963). Figure 1displays the temperature dependence predicted for the model. The activation energies noted on the diagram are in line with the assumption of kinetic control. The derived rate constants are thus given by
0 "3 -60
I
I
1
I
I
-40
-20
0
20
40
lo'
exp(-24448/RTK)
(12) (13)
1.75
-'
& 0
E
.-EMMA
60
k , = 9.77 X 1.67
60 I
2m
I3
70
kl = 1.88 X lo9 eXp(-29451/RTK)
Figure 1. Temperature dependence of reforming rate constants.
40
f
.................. n:...............,,,,A
-
29.5
1OOOTT, K
u
................... A ...................................
80
I 60
RATE OBSERVED
Figure 2. Global comparison of model to data.
reaction j ; rj = rate of reaction j , g-mol/(s.g of catalyst); and nr = number of reactions. The rate of each reaction is dependent on temperature
k3 = 3.33 X 10" exp(-34063/RTK) (14) where k, = rate constant for reaction j , LZ/g-mol.g cata1yst.s); R = gas constant, 1.9872 cal/(g-mo1.K);and TK = temperature, K. Figure 2 compares the model predictions, for component reaction rates, to the experimental values over the entire range of collected data. This set of data represents amine species reforming conversion from 7 to 54 % , providing a good span of the kinetic region. While some scatter is evident, there are no areas of obvious disagreement. Figures 3 and 4 compare the predicted species distribution (both at fixed reactor temperature of 300 "C and feed composition) at various space velocities,to those measured in the Berty reactor. Overall, it appears that this mathematical model describes the reforming process quite well. The model may now be applied to determine if there are any advantages to running a reforming process under kinetic control. Figure 5 shows the results of a simulation of MMA reforming at 350 "C in a plug flow reactor under kinetic control. While the exit (equilibrium condition) contains mostly TMA, early in the reaction sequence the preferred product is DMA. The incremental DMA/TMA selectivity falls as the MMA conversion increases. While not
184 Ind. Eng. Chem. Res., Vol. 33, No. 2, 1994 100
,
R
0
1000
2000
4000
3000
GHSV, hr
5000
-'
Literature Cited
Figure 4. Model predictions at 300 "C; MMA feed.
-..-,
0
10
DMA selectivity
20
30
40
50
and several space velocities. These data were used to develop a mathematical model, which employs a simple mass action based kinetic functionality, that adequately describes the methylamine reforming process. Application of this model to MMA reforming at 350 "C predicts a maximum in DMA concentration of 18%at approximately 60% MMA conversion. At MMA conversionsgreater than 60%, all of the MMA and some of the DMA are converted to TMA and "3. Operation under kinetic control clearly is necessary to achieve maximum DMA selectivity over this catalyst.
60
70
80
MMA CONVERSION
Figure 5. MMA reforming in a plug flow reactor at 350 OC.
pronounced, there is a maximum DMA concentration of 18% at approximately 60% MMA conversion. Beyond 60 % conversion of MMA, all of the MMA (and even some of the DMA) is converted to TMA and "3. If the objectiveis to convert MMA selectivelyto DMA, operation under kinetic control is certainly preferred. Reforming of TMA/NH3 mixtures shows no advantages of operating under kinetic control.
Deeba, M.; Ambs, W. J.; Cochran, R. N. Methanol Amination. U.S. Patent 4 434 300, 1984. Gibson, T.; Ball, T. M.; Nakamura, E. Alkylamines. In Chemical EconomicsHandbook;Stanford Research Institute: Menlo Park, CA, 1988;p 611.5030. Issoire, J.; van Long. Study of the Chemical Thermodynamics of Methylamine Formation Reactions. C. Bull. SOC.Chim.Fr. 1960, 2004. Leonard Process Co., Inc. Methylamines.HydrocarbonProcess. 1979, 59 (ll),194. Marquardt, D. An Algorithm for Least-Squares Estimation of NonLinear Parameters. SZAM J. 1963,11,431. Restelli, E. F.; Coull, J. Transmethylation Reactions of Monomethyl and Dimethylamine over Montmorillonite in a Flow System. AZChE J. 1966,12 (2),292. Rohm & Haas Company. Methylamines.HydrocarbonProcess. 1965, 44 (ll),241. Schmitz, G. Cinetique de la Synthese de la Monomethylamine (Kinetics for the Synthesis of Monomethylamine).J. Chim. Phys. 1975, 72 (5),579. van Gysel, A. B.; Musin, W. Methylamines. In Ullmann's Encyclopedia of Industrial Chemistry, 5th ed.; VCH Publishers: Deerfield Beach, FL, 1988,Vol. A16, p 535. Weigert, F. J. Preparation of Monomethylamine. U.S. Patent 4 254 061,1981. Weigert,F.J.SelectiveSynthesis and Equilibration of Methylamines on Sodium Mordenite. J. Catal. 1987,103,20.
Received for review October 6 , 1993 Accepted October 19,1993 @
Summary and Conclusions Reforming rates of MMA and DMA over LA-30 silicaalumina catalyst were measured at three temperatures
Abstract published in Advance ACS Abstracts, December 15,1993.
