Kinetic study of mtbe liquid-phase synthesis using C4 olefinic cut

Pergamon

Chemical Engineering Science, Vol. 49. No. 24A, pp. 4563-4578, 1994 Copyright © 1995 Elsevaer Science Ltd

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0009-2509(94)00396-3

KINETIC STUDY OF MTBE LIQUID-PHASE SYNTHESIS USING C 4 OLEFINIC CUT

DAVID PARRA, JAVIER TEJERO,* FIDEL CUNILL, MONTSERRAT IBORRA and JOSI~ F. IZQUIERDO

Chemical Engineering Department, Faculty of Chemistry, University of Barcelona, C/Marti Franqu~s, 1. 08028 - Barcelona, Spain

(Received May 1994; accepted for publication October 1994)

Abstract--The liquid-phase addition of methanol to isobutene to give methyl tert.-butyl ether (MTBE) on the ion exchange resin Bayer Catalyst K2631 has been studied using a C4 olefinic cut as the source of isobutene. Rate data were obtained free of mass transfer influence in a continuous differential reactor operated at 1_5 MPa and 45-90 °C. Data show that isobutene has an enhancing effect on the rate whereas methanol has an inhibitory one. As alcohol--olefin-ether mixtures behave non-ideally, kinetic analysis has been performed by using the UNIFAC liquid-phase activities of isobutene, methanol and MTBE. The experimental data can be described by a three-parameter expression based on an Eley-Rideal mechanism in which methanol, adsorbed on one catalytic center, reacts with isobutene from solution to give the ether adsorbed on one center. Surface reaction is the rate-limiting step, and an additional center takes part in this step. The kinetic model emphasizes the relevant role of MTBE in the reaction, which competes for the active centers with the methanol. An activation energy of 81.1 4-4.2 kJ/mol, in good agreement with literature values, was found.

I N T R O D U C T I O N

Environmental legislation around the world has forced the use of oxygenates for gasoline blending to phase out the lead additives and to reduce the reactive evaporative and exhaust emissions_ The US Clean Air Act Amendments of 1990 set forth the basic guidelines for future transportation fuel composition and require the manufacture of oxygenated and reformulated gasolines (Peeples, 1991; Hadder, 1992; Unzelman, 1993). The use of oxygenated gasoline requires that, beginning 1 November 1992, gasoline with a minimum oxygen content of 2.7 wt% must be sold during winter months in US cities not in compli- ance with CO standards. The reformulated gasolines (RFG) with a minimum of 2 wt% oxygen are required by 1 January 1995, in nine areas with extreme or severe pollution problems. Taking the surrounding areas into consideration, the minimum market for RFG in 1995 is expected to be 30% of the US current market. Besides, other areas with less severe ozone and CO problems can opt-in to the oxygenated and RFG programs. This fact could account for an additional 20-35% of current gasoline demand. As a result, the total market for RFG could exceed 50% by 1996 in U.S.A. Meantime, the European Commission has suggested the introduction in Europe of a lightly reformulated gasoline ("clean gasoline"), with lower volatility and benzene levels and with the presence of oxygenates, until catalyst cars become the norm across the Community (10-15 years). Consequently, the global demand of oxyge-

*To whom correspondence should be addressed.

nated organics will increase enormously in the years ahead.

Nowadays, the attention of the petroleum industry is focused on the question of which ether (tertiary) or alcohol (methanol, ethanol, tert_-butyl alcohol), or their mixtures can do the best job of handling RFG blending within the refinery complex and at a lower cost. Tertiary ethers, such as methyl tert_-butyl ether (MTBE), ethyl tert.-butyl ether (ETBE) and tert.- amyl methyl ether (TAME), are the preferred compounds, for among other reasons, because of their lower Reid vapor pressure (bRvp), lower vaporization latent heats, and because they avoid phase separation in the presence of water, which accounts for their full compatibility with the petroleum refining and distribution systems. For a variety of reasons, mainly economic, MTBE has overwhelmingly become the oxygenate of choice. Thus, global MTBE production and use will achieve the highest growth rate of all major chemicals during the 1990s. MTBE demand is forecasted to be tripled (29 million metric tonnes) by the year 2000 (Hunt, 1993).

Commercially, MTBE is typically produced by the reversible liquid phase addition of methanol to isobutene on sulfonated macroporous ion-exchange resins as catalysts (mainly, Lewatit SPC 118, Amberlyst 15 and Purolite CT-175), the C4 olefinic cuts from catalytic (CC) or steam (SC) cracking being the source of isobutene feedstock. The reaction is moderately exothermic and very selective in the presence of these catalysts. However, some byproducts such as dimethyl ether, methyl sec.-butyl ether--by the addition of methanol to linear butenes (Kaitale et al.,

4563

4564 DAVID PARRA el al.

1988; Sarathy and Suffridge, 1993; Vila et al., 1994)- and diisobutene can appear if the temperature is high enough and the molar isobutene/methanol ratio (R) is far from stoichiometric_ The presence of tert.-butyl alcohol is also possible if the reactor feed contains water and/or dimethyl ether is formed.

Typical plants work at 30-100 °C and 0.7-1.4 MPa wtih isobutene conversions higher than 98% (Hutchings et al., 1992). At the sight of the high isobutene conversions of MTBE plants, one can realize the relevant role of the thermodynamics and kinetics of MTBE synthesis in the accurate design of the reactor. The thermodynamics of the reaction has recently been the object of a sizeable effort, and the thermodynamic parameters of the reaction (A/-/~,~_98 = -37.3 + 2.0 kJ/mol, AGr°298 = -14.1 -t- 2.0 kJ/mol, AS~r,298=-79.8+l.0J/(molK ) (Izquierdo et al., 1994)), and the temperature dependence of the equilibrium constant are well established. The non-ideality of methanol-MTBE-C4 stream mixtures has been considered for a long time in thermodynamic studies, and equilibrium constants have been computed from the activities of the compounds at equilibrium, the activity coefficients being evaluated through the UNIFAC method (Colombo et al., 1983; Gicquel and Torck, 1983; AI-Jarallah et al., 1988; Izquierdo et al., 1992, 1994; Cunill et al., 1993)_ The temperature dependence of the equilibrium constant is given by (Izquierdo et al., 1994):

K = 1.65 × 10-4exp[4224.34/T]. (1)

As for the kinetics in the presence of ion-exchange resins, several papers can be found in the open literature (Ancillotti et al., 1977, 1978; Muja et al., 1982; Gicquel and Torck, 1983; Colombo and Dalloro, 1983; Imaizumi and Fujiwara, 1984; Voloch et al., 1986; Subramaniam and Bhatia, 1987; Kaitale et al_, 1988; A1-Jarallah et al., 1988; Ali and Bhatia, 1990; Rehfinger and Hoffmann, 1990). Between them, the following should be emphasized because of their contribution to the knowledge of the reaction.

The first work is that of Ancillotti et al. (1977). These authors used SC C4 streams as the source of isobutene, and conducted their experiments in a batch reactor at 60-80°C, overpressure to maintain the mixture in the liquid phase, and with Amberlyst 15 as the catalyst. From the initial rate measurements, they conclude that MTBE synthesis has a zero order dependence on methanol concentration ( C M ~ ) f o r CMe > 4 m o l / I (wi th negative orders at lower CM~), a first order dependence on isobutene concentration (CIB), and a dependence of about third order on acid group concentration. An activation energy of about 71kJ/mol was also found. In the second work, Ancillotti et al. (1978) explored the role of R on the reaction rate at 60 °C using isobutene diluted with l-butene. At R < 1.7, the initial rates show a zero order in methanol and a first order in isobutene. On increasing R (R > 1.7) the rate increases when CM, decreases, until a maximum rate is reached. A further reduction in CM~ leads to zero

order in isobutene and first order in methanol (R > 10). The rates reported in the second work are higher than those of the first one by 60% (Fig. 3, later). As a result, the question of whether the composition of C4 cuts influence the catalytic activity arises. Finally, it must be pointed out that no kinetic expression was reported.

Later, Colombo and Dalloro (1983) published the first empirical equation rate for MTBE synthesis. Steady state rates were measured in a continuous recirculated fixed bed reactor at 1.4MPa and 40- 70 °C. A SC Ca olefinic cut was the source of isobutene, and the resins Amberlyst 15, Kastel C 350 P and Lewatit SPC 118 were used as catalysts. The rates found for all the resins were similar, but lower than those of Ancillotti et al. (1977) by 20% (Fig_ 3, later), probably due to the different experimental techniques used. The rates were correlated with the average concentrations in the reactor and, at 60 °C, the best rate equation, valid for R < 8 and catalyst acidities > 3.47 meq H ÷/g, was:

r(mmol/eqH + s) = 1 Naflgc-°.9492t"-0-2319g "-0.6593 • . v J v ~ [B ~Me ~ A C "

(2)

An almost zero order in MTBE was found, and apparent activation energies of 74,9, (Amberlyst 15), 79.1 (Kastel C 350P), and 79,1 k J/tool (Lewatit SPC 118) were reported.

Simultaneously, Gicquel and Torck (1983) published the first Langmuir-Hinshelwood (LH) rate model for the reaction. Integral experiments of synthesis and decomposition of MTBE were done on Amberlyst 15 in a thermostated batch reactor operating at 50-95 °C, CMe < 5-3 mol / l (0.75 < R < 1.92), and using a C4 cut as the source o f isobutene. They conclude that the fo l lowing mode l adequate ly represents the rate data:

kfCIBCMe - kbRK CMT

CMe -t- R K C M T

This model stems from a mechanism in which isobutene from the solution reacts with the methanol adsorbed on the resin. Only an active center takes part in the rate-limiting step of the reaction_ An activation energy of 82.0-F6.TkJ/mol is given. Unfortunately, no rate data were reported, only rela- tive values of the rate coefficients and equilibrium constants. Finally, it is striking that the kinetic study was done on the basis of the concentration of species after previously studying the chemical equilibrium on the reaction by means of activities.

Recently, Rehfinger and Hoffmann (1990) have reported an LH rate expression as a function of activities. They studied the synthesis of MTBE in a CSTR at 50-90 °C and 2.0 MPa in the presence of Amberlyst 15 and a CVT resin, made on request, as catalysts. They used pure isobutene diluted with n-butane and l-butene where appropriate, and found that rate data, similar for both resins, were unaffected by the internal mass transport for particles of less than 0.1 mm in

Kinetic study of MTBE liquid-phase synthesis using C4 olefinic cut 4565

diameter and high values of CM~. Reported data agree fairly well with those of Ancillotti et al. (1978) up to Cta of 5 mol/l, and are higher than those of Ancillotti et al. (1977) and Colombo and Dalloro (1983), as can be seen in Fig. 3 (later)_ The main kinetic finding is the linear effect of Cm but only at constant CMe. Moreover, they pointed out that increasing CMe by decreasing the 1-butene causes a slight positive effect on the rate but the reason remains unclear. The following rate expression adequately represents their rate data.

KIB (aIB kbKMTaMT x r ---- kr ~ \aMe krKMeKiaa~,leJ. (3)

The model stems from a mechanism in which isobutene and methanol adsorbed on the resin react to give MTBE, two active centers taking part in the rate- limiting step. An activation energy of 82.4 kJ/mol is given. However, it should be emphasized that, despite the kinetic model proposed, the adsorption enthalpy ofisobutene was not taken into account in computing the activation energy.

Nowadays, some questions still remain unclear. From the refining industry standpoint, a rate equation as a function of activities obtained in the presence of C4 streams would be of interest. Moreover, according to Colombo and Dalloro (1983), and Rehfinger and Hoffmann (1990) the kinetic role of MTBE is to enhance the reverse reaction. On the other hand, Gicquel and Torck (1983) conclude that the ether plays a more important role, and adsorbs up to a point on the resin competing with methanol for the active centers. To sum up, what is the actual role of the ether in MTBE synthesis? Are the discrepancies due to the different experimental procedures and sources of isobutene used? Or perhaps are they attri- butable to the different kinetic analysis performed? Accordingly, with the aim of contributing to the answer of these questions the present kinetic study using SC C4 olefinic cuts was undertaken.

EXPERIMENTAL SECTION

Materials Methanol and MTBE HPLC (<0.01% water,

99.9% pure) were supplied by Romil Chemicals Ltd (Shepshed, UK). Isobutene was supplied as a Ca- hydrocarbon stream of a SC unit containing isobutene (40-55 wt%), 1-butene (20-30 wt%), n-butane (5-15 wt%), trans-2-butene (5-10 wt%), cis-2-butene (3-8 wt%), isobutane ( < 5 wt%) and other hydrocarbons (< 1 wt°/o). The catalyst was the ion-exchange resin Bayer Catalyst K2631 (formerly, Lewatit SPC 118). The nominal degree of crosslinking, BET surface area and exchange capacity of the resin were 18 wt% DVB, 36 m2/g and 4.83 meq/g, respectively.

Apparatus Figure 1 shows a schematic diagram of the setup

made of stainless steel. The reaction was studied in a continuous upflow packed-bed microreactor (15cm

length; 4.4mm i.d.) (9)_ Methanol (or methanol- MTBE blends where appropriate) was fed from an atmospheric tank (1), and the Ca-hydrocarbon stream from a pressurized bottle (2). Filters (21.tm) (3) were placed in the suction lines in order to avoid plugging. A positive displacement pump duplex-type (4), accuracy +0.3% flow rate at pressures over 0.7MPa, dosed independently with methanol (or methanol-MTBE blends), and a Ca-hydrocarbon stream, fed the reactor_ The feeding lines were equipped with pulsation dampers (5) to supply an almost constant flow, and safety (6), and check (7) valves. The feed was homogenized in a mixer (3/8 in o.d.; 18cm length) (8) packed with l mm Pyrex spheres, preheated in a preheater coil (10), and then fed to the reactor_ The reactor temperature was controlled by immersing the reactor in a thermostatic bath (11), thereby ensuring a temperature constancy of +0.1 K. Temperature readings were taken with three thermocouples (accuracy +0.5K) placed at 2.5, 7.5 and 12.5cm from the reactor inlet_ Two sam- ple valves (12), preceded by 21am filters (3), allowed samples to be taken from the reactor inlet and outlet streams to be analyzed. The reactor operating pressure (about 1.5 MPa) was controlled by a back pressure regulator (13).

Analysis The sampling valves injected 0.21al into an HP

5890A GLC apparatus equipped with FID. A 50m x 0.2mm x 0.51.tm methyl silicone capillary column (PONA-HP 19091S-001) was used to sepa- rate and determine the MTBE, methanol, and the reaction byproducts (diisobutene, dimethyl ether and tert.-butyl alcohol). The column was temperature programmed with a I min initial hold at 0 °C followed by a 2 K/min ramp up to I0 °C, and held for 2 min. Then, a 20°C/rain second ramp up to 160°C, held for 8min. The C4 mixture analysis was done on a 50m x 0.32mm x 51am A1203 fused silica capillary column (PLOT-HP 19091P-AL5). The PLOT column was temperature programmed with a 4min initial hold at 65 °C followed by a 9 °C/min ramp up to 190°C. In both cases, helium (SEO, Barcelona) with a minimum purity of 99.998% was used as a carrier gas. The carrier gas flow rate was 30 ml/min.

Procedure The catalyst was crushed and sieved, afterwards

dried at I I0°C for more than 14h, and stored in a desiccator over concentrated sulfuric acid (98 wt%). The water content in the resin, measured by Karl- Fischer titration, was less than 2.9 wt%. By grinding and sizing, prepared particle size distributions were characterized after drying by their geometric mean sphere volume equivalent diameter_

Catalyst samples were diluted in quartz to obtain an isothermal bed and a good contact pattern between reactants and catalyst, avoiding back-mix- ing and channeling. The remaining reactor volume was filled up with quartz of the same size to avoid

4566 DAVID PARRA et al.

_n= 3

I 3 I

!

Fig. 1. Experimental setup: 1, atmospheric tank; 2, pressurized bottle; 3, 2 ttm filter; 4, pump; 5, pressure damper; 6, safety valve; 7, check valve; 8, mixer; 9, reactor; 10, thermostatic bath; 11, preheater coil; 12, sampling valve; 13, back-pressure

regulator; 14, gas chromatograph.

bed expansion. Before the experiment, the catalyst bed (8-13 cm length) was preheated in the reactor at the operating temperature in the presence of the methanol stream (70 ml/h for 30 min), to ensure that an amount of methanol greater than 10 times the reactor volume was passed through it. As a result, the water content in the resin was reduced to less than 1.6wt% according to Iborra et al. (1993).

The experiments were carried out far from limita- tion by chemical equilibrium. Steady-state conversions lower than 10% were measured for the methanol-C4 cut, and the methanol-C4 cut-MTBE feeds. When the reactor behaved differentially, the radial and axial gradients of temperature were lower than 1 °C, and byproducts were not detected in the reactor effluent. Moreover, according to the dilution criterion of van de Bleck et al. (1969), observed rates deviated less than 5% from isothermal intrinsic ones. As a result, with consideration of reaction stoichio- metry, the reaction rate was computed from methanol conversion by

r = W = W I. \xMe + xr~-r} o

~,XMe "l- XMT/i

In each run, the reactants flow rates were main- tained for 3--4 h, and repeated analyses of the reactor

inlet and outlet streams were done to assure that steady state was reached. Reported rates are accurate to within +3°/0. More detailed information about setup and procedure can be found elsewhere (Fit~ et

al. , 1994)_

RESULTS

External and internal mass transfer effects A series of blank runs with the reactor packed with

quartz showed that the reaction did not occur in the absence of catalyst. The influence of the internal mass transfer on the rate was studied by carrying out experiments where, at the same flow rate and feed composition, catalyst samples of different particle size were used. Data showed that the medium polarity determines the largest particle size working in a kinetic regime. As Fig. 2 shows, at 89.2 °C the rates were similar, within the limits of experimental error, for particles smaller than 0.40mm at R = 0.91, and for particles smaller than 0.16mm at R = 0.67. The fact that mass transfer control appears at the smaller catalyst particles is unexpected. However, the effect of particle size on the rate of MTBE formation is very complex and deserves additional specific studies. Previously, Rehfinger and Hoffmann (1990) reported that on lowering CMe, in a limited region of concentrations, larger particles are more effective than smaller ones. One could expect that both experimental results are due to the same physicochemical

Kinetic study of MTBE liquid-phase synthesis using C4 oleflnic cut

1.6

1 . 4 -

1.2-

A - - - 1 o )

4z

_ ~ 0.8 o E "E" o.e-

0.4-

0.2-

0 o 2

f f

• . 1

~. ~ 3

~1 6 8 1'0 1'2 1/dpm (mm ^ (-1))

~ E

1'4 1'6 1'8 20

4567

Fig. 2. Influence of internal mass transport on the rate (Series 1: 89.2°C; R 0.91; LHSV 94h -I. Series 2: 89.7°C; R 0.67; LHSV l16h -~. Series 3: 59.6°C; R 0.91; LHSV 87h-~).

principles. On the other hand, Fig. 2 also shows the effect of temperature on the influence of internal mass transfer is practically negligible. As a result, to avoid internal mass transport effects on the intrinsic rate, and obtain isothermal measurements, particles smaller than 0.1ram were selected in the subsequent kinetic study. Finally, in order to avoid external mass transfer effects and on the basis of the authors ' previous study of ETBE synthesis in the same experimental setup (Fit~ et al., 1994), total flow rates equivalent to LHSV > 60h -] were used.

Kinetic experiments

Kinetic experiments were carried out at the operating conditions given in Table 1, at the average tem- peratures of 89.2, 74.3, 59.6 and 44_6°C. At each temperature three experimental series were per- formed_ In the first one pure methanol was fed, and in the others me thano l -MTBE blends containing up to 60 mol% M T B E (RMT/M E up tO 1_5) were used.

The presence of water in the system was carefully controlled because of its inhibitory effect on the rate (Buttersack, 1988; Cunill et al_, 1993). Besides, water seriously affects the catalytic activity of resins since its

Table 1_ Operating conditions

Catalyst particle size, mm < 0.1 Ca talyst mass (dry). g 0.01 q). 16 Inert particle size, mm 0.1 <dp < 0.16 Temperature, °C 45-90 Pressure, MPa 1.5 LHSV, h -t > 60 R 0.24-3.62 Isobutene concentration, mol/l 1.45-4.75 Methanol concentration, mol/I 0.95-11.1 MTBE concentration, mol/l 0-4.06 Limiting reactant conversion, % < 10

C~S 4g:2~-T

presence changes the medium polarity and, as a result, the resin structure. Therefore, the water concentration was kept to a minimum in order to measure coherent rates within the limits of experimental error. Dried methanol was used, and the water content of the resin was reduced by percolating the methanol stream through the bed prior to experiments (Iborra et al., 1993). At the steady state, water adsorbed in the catalyst was at equilibrium with the water in the feed (<0.03 wt%). After 3h one can expect that the water of the resin was washed out by isobutene as tert.-butyl alcohol. As tert.-butyl alcohol was not detected at the steady state, the amount of water in the resin was very low, and its effect on the rate negligible.

Figure 3 shows the experimental rates at 59.6 °C. As can be seen, the data are similar to the reaction rates reported by Ancillotti et al. (1977), and those computed by eq. (2), but lower than the rates reported by Ancillotti et al. (1978), and those computed by eq. (3). Considering that Ancillotti et al.

(1978) and Rehfinger and Hoffmann (1990) used isobutene diluted with 1-butene instead of C4 streams, it seems that C4 hydrocarbons other than isobutene and l-butene play an important role in the kinetics of MTBE synthesis influencing the catalytic activity of the resin. Taking into account that Colombo and Dalloro (1983) obtained integral isobutene conversions (up to 80%) we can assume that the rates obtained are representative of the behavior of a reactor that uses C4 stream as the isobutene s o u r c e .

From rate data one can draw the following conclusions:

1. In all the series, the reaction rate increases with R (Fig. 4).

4568

0,3

0,25

0,2

0,15

0,1

0,05

DAVID PARI~-A el al.

r ( m o l l ( h ' g ) )

4. ° o o ° o

° ' ' ' " 1 •

, • • ~ J ' q

0 0,5 1 1,5 2 2,5 3 3,5 4 4,5 5

C IB (tool/I)

Fig. 3. MTBE synthesis at 59.6°C. Rate vs isobutene concentration (ll , RMT/Me = 0; + , RMT/M e = 0,563; O, RMT/M¢ = 1.44). 1, results of Ancillotti et al. (1977); 2, results of Ancillotti et al. (1978); 3, results predicted by eq. (2)

(Colombo and Dalloro, 1983); 4, results predicted by eq, (3) (Rehfinger and Hoffmann, 1990)_

0.25-

0.2-

0.18"

~ 0.1

0.08

0 0:5 i 1:5 2 2:5 ~1 3:11 4 R (tool II/rnol Me)

Fig. 4. MTBE synthesis at 59.6 °C. Rate vs R (m RMT/Me = 0; +, RMT/Mc = 0.563; x, RMT/M e = 1.44).

2. As can be seen in Fig_ 5, rate decreases on increasing CM= for CM= > 1 mol/l at R values > 1.7 whereas at lower R values, there are no experimental data from which to draw any con- clusion. On the contrary (Fig. 3), the reaction rate increases almost linearly with Cxs, The same trend has been observed at the other tem- peratures. Finally, the reaction rate decreases on increasing CMT (Fig. 6).

3. Temperature highly influences the rate. It increases by three times when the temperature increases 15 °C (Fig. 7).

Non-ideality o f methanol-C4 hydrocarbons -MTBE mixtures

Nowadays, the fact that methanol- isobutene liquid mixtures behave non-ideally is beyond all doubt. As a consequence, the analysis of the methanol-isobutene-- MTBE system should be done by using the activities rather than the concentrations of the substances involved. The activity of component i is related to its molar fraction in the mixture by means of

ai = X~ - "Ti

where the activity coefficients ")'i depend on temperature and composition (3'i = l when xi = 1). The effect

Kinetic study of MTBE liquid-phase synthesis using C4 olefinic cut

0.28-

4569

0.2-

A

0.18-

0 E 0.1-

0.08-

0 0 2 4 6 8 1'0 12

C Me (tool/I)

Fig. 5. MTBE synthesis at 59_6°C. Rate vs methanol concentration (11, RMT/Me=0; +, RMT/Me = 0_563; x, RMT/M © = 1.4.4).

0.25-

0.2-

A

A

o 0.18-

o E 0.1 ~2

0.08

- I -

0 2 4 6 8 1'0 12 C MT (tool/I)

Fig. 6_ MTBE synthesis at 59.6°C. Rate vs MTBE concentration (+, RMT/c =0.563; X, RMT/Mc = 1.44).

of pressure was considered negligible. Estimates of the activity coefficients were made through the UNIFAC predictive method (Reid et al., 1987). The parameters needed for the use of UNIFAC, i.e. group volumes, group surface areas and group interaction parameters were taken from the tables published by Fredeslund et

aL (1977), Skjold-Jorgensen et aL (1979), AImeida el aL (1983) and Tiegs et aL (1987).

Besides methanol, isobutene and MTBE, the actual components of the reaction system are l-butene, n- butane, trans- and cis-2-butene, isobutane, and minor components of C4 streams. Colombo et al. (1983) on studying the chemical equilibrium of the

reaction showed that despite the complexity of the mixture, the predicted activity coefficients of all the hydrocarbon-methanol and hydrocarbon-MTBE binary systems are very similar, and the binary systems between the hydrocarbons are nearly ideal. Therefore, only four components were considered: isobutene, methanol, MTBE and 1-butene in place of C4 hydrocarbons other than isobutene. Estimates of the methanol and isobutene activity coefficients are clearly > l, and increase when their molar fractions decrease, as expected. Their values agree with the calculated and experimental values reported by Colombo et al. (1983) the range of 3' being quite


A

D 41

m

o E im

1.8-

1.6-

1.4-

1.2-

1

0.8

0.6-

0.4-

0.2-

El

Y

0 0:8 i 1:§ i 2 . 8

Fig. 7. Rate of MTBE synthesis vs temperature (11, 89.2 °C; A, 74 3 °C; +, 59.6 °C; I-7, 44.6 °C).

wide ( - ~ : 1.22-6.25; ~a : 1.09-2.89). The MTBE coefficients are close to 1 ('rM-r: 0.926--1.14). Therefore, the ether behaves almost ideally. Finally, the l-butene coefficients are similar to those of isobutene ('~B: 1.08-2.82) in line with previous work of Colombo et al. (1983).

The stronger dependence of rate on activities than on concentrations, within the range explored should be noted, as can be seen in Figs 8-10, which emphasizes the non-ideality of the mixtures. The aMe remains practically unchanged within each series but decreases when the MTBE/methanol ratio in the feed increases. In this way, the change in the methanol concentration appears to be balanced by the subsequent change in the activity coefficient, and aM~ only shows a little variation within each series. Similar behavior can be observed for am in the absence of ether (am > 0.4), On the other hand, the dependence of reaction rate on aM T is similar to that of CMT, showing the ideal behavior of MTBE.

SELECTION OF KINETIC MODEL

Before formulating kinetic models for the reaction, the response surface of the rate as a function of am, aMe, aMT and ata was analyzed. Analysis of the response surface allows one to outline the main features of the kinetic models that best fit rate data,

which saves substantial effort in their formulation and fit. Thus, following the surface response methodology (Tejero et al., 1989; Iborra et al., 1992), a polynomial as a function of ala, aMe, aMT and ala was fitted to data, at 89.2 and 59.6 °C, by a multivari- able least squares method. The expressions in Table 2 are the lower order polynomials that fit the data in a significant fashion for a probability level of 95%. However, although ala, aM~, aM'r and ala are inter- dependent since C[a, CMe, CMT and C i , are not inde- pendent variables (no diluents were added in the feed), the interactive terms were not considered because of the lack of sufficient experimental data. As a result, the analysis of polynomials can lead to some discrepancies (mathematical in nature) in Figs 8-10. Notwithstanding that, taking into account jointly Figs 8-10 and the polynomials, we can draw the following conclusions:

The ala enhances the reaction in all the explored range of am (0.25-0_49). The inhibitory effect for am < 0.46 suggested at 89.2 °C by the polynomial does not have physical meaning because experimental data show an increasing relation- ship between r and ala (the same as at 59.2 °C). Moreover, at RMT/M e = 0, r = 0 for CMe = 0 (ala = 1) a maximum could be expected in the r vs am curve (but out of the explored range of

Table 2. Surface response as a function of activities

T = 89.2 °C r = --7.34aMT + 65.0am + 89.4als -- 79.Ta~T -- 349a~a

-- 279a2, -- 4.08a~c + 89.4a~T + 406a~, + 262a~s

T = 59.6 °C r = --4.48aM~ + 4.15aMT + 4.15am + 13. I am + 3.29a2e

-- 19.5a~T -- 58.2a~n + 24.0a~t T -- 3.52a~B + 64.7a~a


0.25

4571

0 . 2 -

A A

o 0.18- I

,E

E 0 . 1 -

0.08-

v0 0~1 012 0~3 0~4 015 0.6 a lB

Fig_ 8. MTBE synthesis at 59.6 °C. Rate vs isobutene activity (1 , RMT/Me = 0; A , RMT/M e = 0.563; O, RMT/Mc = 1.44).

0.28 -

0 . 2 -

A A

D 0.18- ,11

o E 0 . l - b.

o.o8-

0 i i

o o:2 0.2 04 o15 0'.6 o17 o19 a Me

Fig. 9. MTBE synthesis at 59.6 °C. Rate vs methanol activity (11, RMT/Me = 0; + , RMT/M e = 0.563; x, RMT/M e = 1.44).

activities). In the same way, a maximum could be expected for the other curves.

2. At 89.2 °CaMc inhibits the reaction rate in all the explored range of aMc (0.55-0_83), whereas at 59.6°C aM~ enhances the reaction for a ~ > 0.68 (unlike CM~ (Fig. 5)). In addition, since r = 0 for CM~ = 0 (aM~ = 0) a maximum could be expected in the r vs aMe curve at RUT~Me 1.44. The other curves would probably have similar shapes.

3. The aMT shows an inhibitory effect on the rate in all the range of aM'r (0.125-0.37).

It has been firmly stated that several sulfonic groups take part in the rate-limiting step of ion- exhange catalyzed reactions. In this way, Wesley and Gates (1974) reported that up to seven active centers could take part to stabilize the reaction inter- mediate in the benzene propylation. As for MTBE liquid-phase synthesis, Ancillotti et al. (1977) pointed out a third-order dependence of the rate on the concentration of sulfonic groups for isobutene--methanol equimolar mixtures, and a fourth-order one when isobutene is in excess. However, models found in the literature generally show lower-order dependences.

4572 DAVID PARKA et al.

0.28-

0.2-

0.18"

m

E~ 0.1"

0.05

0 0 011 012 0'.3 014 015 0'.6 017 018 019 a MT

Fig. 10. MTBE synthesis at 59.6 °C. Rate vs MTBE activity (A, RMT/M e = 0.563; ×, RMT/M e = 1.44)_

Thus, Rehfinger and Hoffmann (1990) reported a second-order dependence, whereas Gicquel and Torck (1983) reported a first-order one. The excep- tion is Colombo and Dalloro (1983) who proposed an empirical dependence of about three for catalyst activities > 2.5 meq H ÷/g.

Furthermore, whereas methanol adsorption on the resin is beyond all doubt, isobutene has not been considered in the open literature emphasizing its very low adsorption equilibrium constant (Rehfinger and Hoffmann, 1990). In fact, MTBE synthesis has been frequently described in terms of an Eley-Rideal (ER) mechanism in which isobutene from solution reacts with the methanol adsorbed on the resin (Gicquel and Torck, 1983; Subramaniam and Bhatia, 1987; Ali and Bhatia, 1990). Still, this picture can be reasonably regarded as an oversimplification useful to explain the behavior of the olefin-alcohol mixtures when alcohol is in excess. Otherwise, it is difficult to explain diisobutene formation as a bypro- duct without assuming that isobutene adsorption takes place even if only weakly.

From an eclectic standpoint, and considering the features of the surface response outlined above, we can assume that the best kinetic model for the reaction could stem from the Langmuir-Hinshelwood- Hougen-Watson (LHHW) mechanism that follows:

M e + a = M e - a

I B + a = I B - a

M e . a + I B . a + ( n - 2 ) . ~ r ~ M T . ~ r + ( n - 1) -a

MT-cr ~- M T + a

in which the adsorbed methanol and isobutene react to yield MTBE. Or else, from the following ER mechanism in which isobutene from solution reacts with the adsorbed methanol.

M e + ~ M e - a

Me- or+ IB + ( n - 1 ) - o ' ~ M T - a + ( n - 1 ) .o

M T - a ~ M T + a .

One or more supplementary active sites could take part in the surface reaction step, in both mechanisms. Considering this step as the rate-limiting one, the basic LHHW and ER kinetic models would be, respectively, as follows:

kfKIBKMc( alBaMe -- aMT / K) f - -

(1 + Klsam + KMeaMe + KMTaMT + KIBaIS)"

kfKMe( alBaMe -- aMT/K) r z

(1 -4- KMeaMe q'- KMTaMT -1- KIBale) n

Such models, with n > 2, fulfil the response surface features indicated above. The enhancing effect of isobutene on the rate, is because isobutene influences mainly the driving force of hyperbolic models. The inhibitory effect of methanol is because the alcohol influences mainly the adsorption term. The MTBE inhibits the reaction because the presence of the ether causes a decrease in the driving force. The MTBE can also adsorb on the resin competing for the active centers with the methanol. Finally, these models with n > 2 can appropriately describe the presence of a maximum rate.

From a statistical standpoint, the model with the lowest sum of squares of lack of fit and random residuals would be the most suitable. Nonetheless, it would be desirable that the parameters of the model had a physicochemical meaning. Thus, the rate coefficient had to increase with temperature, and adsorption equilibrium constants decrease. In agreement with that, the activation energy had to be positive, and the adsorption enthalpies and entropies negative. Finally, the adsorption entropies should fulfil the rules of Boudart (Carberry, 1976). The model

Kinetic study of MTBE liquid-phase synthesis using C 4 olefinic cut 4573

types of Table 3 were fitted to data through the non- linear least squares method of Box-Kanemasu (Beck and Arnold, 1977). The thermodynamic equilibrium constant K was estimated by means of eq. (1).

Model type 1 considers the adsorption terms of methanol, isobutene, MTBE and I-butene. Because of the high number of parameters, this model showed a lack of convergency, and, as a consequence, some simplified models were tested. Successively, the adsorption terms of isobutene (model type 2), l- butene (model type 3), MTBE (model type 4) were removed assuming that their influence in the adsorption term was negligible. This assumption, which seems reasonable for l-butene and isobutene, is uncertain for MTBE.

Model types 5, 6 and 7, which stem from removing the I in the adsorption term of model types 2, 3 and 4, respectively, were then tested. This removal can be explained assuming that the fraction of free accessible sites is negligible under the reaction conditions_ This assumption could be admissible taking into account that methanol permeates more easily than

the other compounds, being responsible for the resin swelling.

Finally, model types 8, 9, 10 and 11 were tested. These consider that l-butene, whose adsorption is negligible, affects the activity of the catalyst. Its presence would modify the values of the true rate coefficient because of the changes in the polarity of the mixtures, which would influence the swelling of the catalyst. As a consequence, the number of accessible sites could be influenced by the activity of I-butene (C4 hydrocarbon other than isobutene) (Izquierdo et

al., 1993). Successively, the 1 of the adsorption term (model type 9), assuming that the fraction of free sites is negligible, and the adsorption terms of isobutene (model type 10) and MTBE (model type 11), assuming as well that their contribution to the adsorption term is negligible, have been removed.

Only model types 6, 7, 10 and 11 are physico- chemically coherent. However, as Table 4 shows, the best fit of model types 10 and 11 corresponds to m = 0 , whereby these models, when m =0 , become model types 6 and 7, respectively.

Table 3. Equation type tested for liquid-phase synthesis of MTBE (k" = kfKmKMe for an LHHW mechanism and k" = krKMc for an ER mechanism)

Model t y p e Mechanism type Equation

1 LHHW k'(alBaMe -- a M T / K )

r = ( I + KIBalB + KMeaMe + KMTaMT + KIBOIB)"

2 LHHW RE

]('(alBaM© -- a M T / K )

r = ( l + KM©GMe + KMTaMT + KIBOIB)"

3 LHHW RE

k'(alBaMe -- . M T / K )

r = (1 + KMeaMe + KMTaMT) n

4 LHHW RE

k'(alBaMe -- aMT / K ) r =

(1 + KMeaMe) n

5 LHHW RE

k'("IS~Mo -- "MT /K )

r = (KMeaM e Jr KMIQMT + KIBalB) "

6 LHHW RE

k" (.IBaM¢ -- aMT/K)

r = (KMeQM c + KMTOMT), ,

7 LHHW RE

k'(alBOMe -- QMT/K) r =

(KM©OMe) n

8 LHHW k'(dlBQMe -- QMT/K)

r = d ~ ( l + KIBalB + KMeaMe + KMTaMT) n

9 LHHW RE

k'(dlBaM¢ -- QMT/K)

r = O~I~}(KIBalB -I- KMcaMe + KMTaMT)"

10 LHHW RE

k'(OIBOM¢ -- d M T / K ) r = ~ n

dlIB(KMeaM¢ + KMTGMT)

I 1 L H H W R E

k'(alBOMc -- aMT/K) r = a ~ (KMeaM{),,

4574 DAVID PARKA et al.

Table 4. Main features of the fit of kinetic models

Model type Values of m Convergency ~ > 0? Kj as f(T) Observations and n tested

1 n = 2-7 No 2 n = 2-4 Yes No No 3 n = 2 Yes No No 4 n = 2 Yes Yes No 5 n = 2-5 Yes Yes No 6 n = 1-6 Yes Yes Yes

7 n = 1-6 Yes Yes Yes

8 n = 2-4 Yes Yes No 9 n = 2-4 Yes Yes No

m = 0 - 3 10 n = 2 Yes Yes Yes

m = 0-3 11 n = 2 Yes Yes Yes

m = 0 - 2

Minimum sum of squares for n = 2 Minimum sum of squares for n = 2

Minimum sum of squares for m = 0 Minimum sum of squares for m = 0

Moreover , statistical analysis shows tha t the best models for the reaction are model types 6 [similar to that reported by Gicquel and Torck (1983)] and 7 [similar to the model proposed by Rehfinger and Hof fmann (1990)] when n = 2. Model type 6 with n = 2 has the min imum sum of squares of the lack of fit. Besides, the residuals are r andom (Fig. 11). As a consequence, we assume that this model (see Table 3) adequately represents the react ion data:

r ~ k*( aMealB - - aMT / K) aMealB -- aMT / K

(KMeaMe + KMTaMT) 2 (KlaMe + K2aMT) 2"

(4)

Equat ion (4) has three adjustable parameters (k °, KMe and KMT). However, in order to obta in a more easy fit, it was reduced to two adjustable parameters . These are:

0.25

i

0.2 ~

=.~ _0.15

o g 0

"~ 0 . 1 0

0.05-

Ki = KMe/X/'--K; = At e x p ( - B i / R T ) and

K2 = KMT / V/--k -; = A2 e x p ( - B2/ R T).

Figure 12 shows that KI and K2 decrease with increasing temperature following an exponent ia l variation. This could be expected because the apparen t rate coefficient should increase with tempera ture pro- vided that the sensitivity of the actual rate coefficient to tempera ture was greater than that of the adsorption equi l ibr ium constants , and in addi t ion kr and KMe, KMT, and Kin, where appropr ia te , also depend exponential ly on temperature. As a result, eq. (4) can be regarded as coherent ly temperature-dependent_

Equat ion (4) can be derived f rom an E R mechanism (Table 3), the rate-l imiting step being the surface reaction between the me thano l adsorbed and the isobutene from solution. In this case, KI and /('2 are given by

0 0 o.b5 011 0.;5 012 0,22

r exp (mol/(h g))

Fig. I I. Residuals of the proposed model at 59.6 °C (m, RMT/Me ---- 0; +, RMT/M e = 0.563; X, RMT/M c = 1.44).

tt) ci m o E

?


10:

0.1

0.01 2.7 210 219 3 311

1000/T (K--1)

Fig. 12. Temperature dependence of parameters Ki (ll) and K2 (A,)_

3.2

4575

KI = ~ and K2 = x/KMT/krKMe.

From these one can derive the following relationships between the true activation energy and the adsorption enthalpies of methanol and MTBE

E = A ,Mo - 2S, (5)

m/~a,M T = B2 -'[- 0 . 5 ( E + A/-~a,Me). (6)

From eqs (5) and (6), the activation energy and the adsorption enthalpy of MTBE were computed to be 81.1 -I- 4.2 kJ/mol, and - 16.9 -t-- 12 kJ/mol, respec- tively_ Parameters Bi (-42.5kJ/mol) and B2 (-55.5kJ/mol) were found experimentally from the slope of the plots of In Kt and In K2 vs T ' l (Fig. 12). An adsorption enthalpy for methanol of -3.SkJ/mol found by a combination of reaction calorimetry and thermogravimetry (Sold et al., 1994) was used.

As Table 3 also shows, eq. (4) can also be derived from an LHHW mechanism whose rate-limiting step is the surface reaction between both the methanol and isobutene adsorbed on the resin. In this case Ki and K2 are respectively

Ki = x/KMe/krKm and K2 = x/KMT/krKMeKIS

and the following relationships between the true activation energy and the adsorption enthalpies of methanol, MTBE and isobutene, can be derived

= - 2 s , - (7)

/X ,MT = S2 + 0.5(E + A ,U° (8)

Values for the adsorption enthalpy of isobutene were not found in the open literature. As a consequence, these were estimated from the vapor phase adsorption enthalpy by the expression

AH~a,IB(I) : A/~a,IB(g ) -'~ mn~v,l B,

An adsorption enthalpy of -72.6 + 3 kJ/mol was reported in the gas phase synthesis of ETBE on Lewatit SPC 118 (Iborra et al., 1990). As the vaporization enthalpy of isobutene is about 18.8kJ/mol (Gallant, 1968), we can deduce a value of -53.8 + 3 kJ/mol for A/-~,m0), and by means of eqs (7) and (8), the activation energy and the adsorption enthalpy for MTBE were computed to be 135 + 13 kJ/ tool, and -16.8-t- 15 k J/tool, respectively.

The AJ~a,M T values are similar irrespective of the mechanism proposed. They suggest that adsorption of MTBE is rather temperature sensitive although the values for the adsorption constants are probably low. Unfortunately, the A/-/~,M T values were not found in the literature and it was not possible to test the reliability of the authors' value. On the other hand, the values found for the activation energy are very different. A comparison with the activation energy values reported in the literature (Table 5) shows that 135 + 13 k J/tool (LHHW mechanism) is higher than the values found for homogeneous catalysts. On the other hand, 81.1+4.2kJ/mol (ER mechanism) is in good agreement with those reported on ion-exchange resins. Besides, this value is a little lower than those of homogeneous catalysts. Therefore, it seems unlikely that the reaction occurred through an LHHW mechanism, and one can assume that an ER mechanism represents it adequately. However, from an eclectic standpoint it must be recognized that despite the fact that MTBE synthesis mainly occurs through an ER mechanism, it is possible that an LHHW one could also contribute to the reaction. Particularly, when the polarity of the system is low, i.e. in excess of isobutene, or even with a great amount of ether in the reaction system. It


Table 5. Activation energies for MTBE synthesis quoted in the literature

References Catalyst Eapp E (kJ/mol) (kJ/mol)

Ancillotti et al. (1977) Muja et al. (1982) Gicquel and Torck (1983) Colombo and Dalloro (1983) Colombo and Dalloro (1983) Colombo and Dalloro (1983) Voloch et al. (1986) Subramaniam and Bhatia (1987) AI-Jarallah et aL (1988) All and Bhatia (1990) Rehfinger and Hoffmann (1990)

Evans and Halpern (1952) Ancillotti et al. (1977) Gicquel and Torck (1983) Csikos et al. (1980) Gicquel and Torck (1983)

This work. LHHW mechanism This work. ER mechanism

Amberlyst 15 71 No specified resin 68.3 -t- 0.8 Amberlyst 15 82.0 ~- 6.7 Amberlyst 15 74.9 Kastel C 350P 79.1 Lewatit SPC 118 79.1 Amberlyst 15 71 Amberlyst 15 76.7 + 2.0 Amberlyst 15 87.9 Amberlyst 15 68.9 Amberlyst 15 92.4 82.4

p-Toluenesulfonie acid 101.5 p-Toluenesulfonic acid 104.1 p-Toluenesulfonic acid 91.2 Sulfuric acid 85.7 Methyl sulfuric acid 87.9

Bayer K-2631 135 + 13 Bayer K-2631 81.1 ± 4.2

should be pointed out that the fact that the fit of eq. (4) gives the apparent rate coefficients (Ki and /(2) instead of the actual rate coefficient and adsorption equilibrium constants, has hindered a further physicochemical analysis including a comparison of adsorption enthalpies with experimental values (when possible) and testing the fulfilment of the Boudart' rules for the adsorption entropies, which would cast light on the screening of the best model and mechanism for MTBE synthesis. Finally, the MTBE effect on the adsorption term should be emphasized. This fact shows that, according to Gicquel and Torck (1983), MTBE plays an important role in the reaction competing for the catalytic centers.

be concluded that the most probable mechanism is the Eley-Rideal one. The inhibitory effect of MTBE on the rate is mainly due to the adsorption of the ether on the catalyst, competing for the active centers with the methanol.

A t , A 2

ai

BI, B2 Ci

d, E

Eapp

:Mo CONCLUSIONS

Synthesis of MTBE by addition of methanol to IB isobutene has been studied on the ion-exchange K resin Bayer catalyst K2631 at 1.5 MPa and 45-90°C. Rate data show that the reaction is highly tempera- Ki ture-sensitive, and isobutene has an enhancing effect on the rate whereas methanol has an inhibitory one. Kl, K2 Alcohol-ether-hydrocarbon mixtures behave non- kb ideally. Thus, kinetic analysis has been carried out by means of the UNIFAC liquid-phase activities of kr isobutene, methanol and MTBE. The best kinetic model could be deduced from an ER mechanism k" and also from an LHHW one. In the ER mechan- LHSV ism, isobutene from solution reacts with the metha- Me nol, adsorbed on one catalytic center, to give the ether MT adsorbed on one center. In the LHHW mechanism, n the methanol and isobutene, each adsorbed on one center, react to give the ether adsorbed on one cen- R ter. The rate-determining step of both mechanisms is the surface reaction, and two active centers take part RK in this step. From the activation energy values it can

NOTATION

apparent frequency factor, (g h mol-I )0.5 activity of compound i, dimensionless apparent activation energy, kJ/mol concentration of compound i, mol/l particle diameter, mm activation energy, kJ/mol apparent activation energy, kJ/mol molar flow rate of methanol at zero methanol conversion, mol/h isobutene thermodynamic equilibrium constant, dimensionless adsorption equilibrium constant of compound i, dimensionless apparent rate coefficient, (g h/mol) °5 rate coefficient of reverse reaction, mol/(h g) rate coefficient of forward reaction, mol/ (h g) apparent rate coefficient, mol/(h g) liquid hourly space velocity, h -l methanol MTBE number of active centers that take part in surface reaction step isobutene/methanol molar ratio, dimensionless adsorption equilibrium constant of MTBE/methanol ratio, dimensionless

Kinetic study of MTBE liquid-phase synthesis using C4 olefinic cut 4577

RMT/Me

r

T W

Xi

MTBE/methanol molar ratio in the feed, dimensionless intensive reaction rate, mol/(h g) temperature, K catalyst mass, g degree of conversion of reactant i, dimensionless molar fraction of substance i, dimensionless

Greek letters "7i activity coefficient of compound i, dimen-

sionless AG~r standard molar free energy change of reac-

tion, kJ/mol A/-/~ standard molar enthalpy change of reac-

tion, k J/tool A/-/~j standard molar adsorption enthalpy of

compound j, k J/tool A/-/~/ vaporization latent heat of compound j,

kJ/mol A ~ standard molar entropy change of reaction,

J /mol /K A . ~ i degree of conversion change of reactant i,

dimensionless cr active center

Subscripts AC active center a adsorption g gas phase IB isobutene i inlet I liquid phase Me methanol MT methyl tert.-butyl ether o outlet v vaporization 1 B l -butene

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Kinetic study of mtbe liquid-phase synthesis using C4 olefinic cut

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