Developments in the production of methyl tert-butyl ether

Catalysis Today, 15 (1992) 23-49 Elsevier Science Publishers B.V., Amsterdam

23

Developments in the production of methyl teft-butyl ether GJ. Hutchings’, C.P. Nicolaides2 and M.S. Scurrel12 bverhulme Centre for Innovative Catalysis, Department of Chemistry, University of Liverpool, P 0 Box 147, Liverpool IA9 3BX, United Kingdom 2Catalysis Programme, Division of Energy Technology, CSIB, P 0 Box 395, Pretoria 0001, South AIkica

SUMMARY

The catalysis of the etherification of alcohol and iso-olefin mixtures is discussed. Emphasis is placed on the synthesis of MTBE (methyl teti-butyl ether), but mention is also made of other ethers such as ETJ3E (ethyl tert-butyl ether), SAME (see-amyl methyl ether) and MIPE (methyl isopropyl ether) and, to a very limited extent, of hydrocarbon feedstocks other than iso-olefins. Established sulphonated resin catalysts are described together with various aspects of their catalytic characteristics, including kinetics, thermodynamics and the occurrence of side-reactions. The use of alternative inorganic acids, especially xeolites, is covered and comparisons drawn between these systems and the ion-exchange resins. Finally, attention is given to the design of commercial processes for the production of MT-BE. 1.

INTRODUCTION

Of all the oxygenates used or contemplated for use as octane improvers in gasoline, MTElE (methyl terf-butyl ether) has exhibited the highest growth over the past decade [l]. World capacity has increased approximately ten-fold in this period [2,3] and currently stands at about 12 million tons/year, with a projection that this figure will increase to ca 20 million tons/year by 1994. One responsible factor is the phasing out of lead in leaded gasoline, mandated, for example, by the EPA in the United States. The EPA, under the “2 % oxygen in fuel” definition [l], permits up to about 12.7 vol % of MTBE or other oxygenates such as ETBE (ethyl tert-butyl ether) or TAME (tert-amyl methyl ether). There have also been calls for a 2.7 % oxygen level, corresponding to 15 % MTF$E in unleaded gasoline. Oxygenates can also play a role in reducing the levels of pollutants

1992 Ebmier Science Publishers B.V.

24

(especially carbon monoxide) in exhaust emissions, and help to extend the supply of hydrocarbons from crude oil sources. Japan has recently armounced the approval of the addition of up to 7 vol % MTBE to gasoline. Although the commercial production of MTBE via the catalytic combination of methanol and isobutene (equation (l)), using sulphonated ion-exchange resins, is CH3 EI3c

-

&

=

a3 CXl2

+

CH3OH

acid ____c catalyst

H3C! - :: - C!H3 &II

(I)

3

now well established, there are several interesting .aspects now enjoying considerable attention from catalytic scientists and engineers. Among these can be mentioned: - the use of alternative catalysts, such as inorganic acids, or perhaps superacids for the exemption reaction; -

the production of MTBE from syngas, most directly accomplished (at least on paper!) from the development of a mixed isobutanol-methanol synthesis followed by conversion of the isobutanol to isobutene or, even more attractively, by the direct reaction of isobutanol and methanol;

-

the advantageous combination of simultaneous catalytic etherification and distillation in which the catalyst is included in the packing of the distillation tower. This approach is exemplified by the recently announced ETHERMAX process from UOP [4].

Issues closely related to h@I’BE production concern isobutene availability, leading to a search for commercially viable catalytic processes for the skeletal isomerisation of n-butenes [5], and resulting in the application of Phillips’ STAR technology for the production of isobutene form n-butane [6]. Finally, the use of feti-butyl alcohol (TBA) could be contemplated since its gasoline-related properties are close to those of the fuel ethers, but first, a commercially attractive method of producing this alcohol must be developed [7]. This review ~nce~ates on three aspects, namely: -

the catalysis of the etherification reaction using established sulphonated resins,

-

the etherification by alternative catalysts; and

-

the process design/commercial aspects of current and emerging technologies.

26

2.

CATALYSIS OF ETHERIFICATION

USING ESTABLISHED SULPHONATED

RESINS

Since 1973 a plant based on the SnamprogetWEcofuel (formerly ANIC) process and with a production capacity of 100 000 tons of MTBE per year has been operating at RaveItaly [8]. The process, shown in Fig. 1, uses isobutene, which is contained in a C4 fraction obtained from steam cracking or catalytic cracking. After extraction of the butadiene, the C4 feed stream has an isobutene content which ranges between 35 and 53 wt %. The reaction is carried out in a Methanol

T

1

C4 Raffinate b

Butenes Feedstock

t

rt 2 YI t

I_

I

f

Reaction Loop

F’ig. 1.

MTBE Process

MTBE

MTBE Fractionation

Methanol Removal

(Smunprogettel-John

Methanol-Water Fractionation

Brown) [8,2@1.

fixed-bed tubular reactor and uses ion-exchange resins as catalysts. Linear butenes are completely inert under the operating conditions used and by-product production is very limited with formation chiefly of diisobutene. The water present in the reagents reacts with isobutene, producing tert-butyl alcohol. The effluent from the reactor consists mainly of MTBE and linear butenes. This mixture is then sent to the purification column in which the Cd’s are separated from the MTBE.

26

In commercial practice, cation-exchange resins (for example Bowex 5Ow, Amberlyst 15 (AlS), Lewatit SPC 118 or Nacite) which are sulphonated copolymers of styrene and divinylbenzene (DVB), the cross-linking agent, are used as fixed-bed catalysts for the etherification reaction. These resins are based on the bead forms of this copolymer which were developed in the late 1940’s and early 1950’s [9]. Amberlyst 15, produced by Rohm and Haas, is a strongly acidic macroreticular resin and contains 28 % divinylbenzene. The resin has a surface area of 43 k 1 m2/g and a mean pore diameter of 240 8, The ion-exchange capacity determined by titration is 4.8 mequiv. of HSO3/g of dry resin [lo]. The catalyst life in commercial operation has been found to be in excess of two years. The addition of methanol to isobutene is both highly exothermic (AH&a = -37.7 kJ/mol) and reversible and the process is carried out in a multi-tubular reactor with recirculating water between the tubes in order to remove reaction heat. Using a slight excess of methanol in the feed, the per pass conversions of isobutene can be close to 100 % [ll]. The reaction temperatures employed range between 30 and 100 “C and the pressure between 7 and 14 atm [12]. Increased temperature shifts the equilibrium of the process toward the starting materials, producing a faster reaction at the expense of net conversion, the life of the catalyst, and the loss of isobutene due to polymerization [13]. The first research report on the use of an ion-exchange resin, in this case Amberlyst 15, as catalyst for the addition reaction of tertiary olefins and alcohols to produce ethers was published in 1977 by researchers at Snamprogetti [14]. In their experiments, which were conducted at 60 “C in an autoclave provided with a mechanical stirrer, the ion-exchange resin was found to display a higher activity for MTBE synthesis than the soluble anhydrous p-toluene sulphonic acid. Liquid-phase mass-transfer resistance was negligible and no induction periods were observed. From the initial reaction rates, the reactivity of primary alcohols with isobutene was observed to follow the order n-butanol > n-propanol > ethanol > methanol. Their results also showed a zero order dependence of rate on methanol concentration for concentrations greater than 4 mov1, with negative orders at lower concentrations (see Fig. 2) and a first order dependence on isobutene concentration. A strong dependence of rate on acid group concentration, about third order, was also observed. Studies on the reactivity of three isomeric isoamylenes with methanol showed that 2-methyl-1-butene and 2-methyl-2-butene can undergo both the etherification and double-bond-shift isomerization reactions whereas 3-methyl-1-butene does not react at all. These results suggest that a carbenium ion is the common intermediate and that olefin protonation is a more important kinetic step than the

21

25-

51

Fig. 2.

2

3

4

5

6

7

Methanol,initial amcentration,md/litre

Methyl teti-butyl ether synthesis. Initial rate dependence on initial methanol concentrations at constant isobutene content (4 mol/l). Temperature: 60 “C; catalyst: Amberlyst 15 [14].

interaction with the nucleophile. The observed high order in the 40$-I groups can be related to a non-linear dependence of protonating power of the resin on the concentration of sulphonic groups. In a subsequent paper from the same group [15], the role of the isobutene/alcohol ratio, a, on the reaction mechanism was further explored. It was found that the initial rates of the reaction between methanol or n-butanol and isobutene show different mechanisms, depending on the ratio of the reactants. At stoichiometric or lower isobutene/alcohol ratios (a < 1.7, see Fig. 3), the initial rates show a zero order in the alcohol and a first order in the olefins. In this case the experimental data agree with an ionic mechanism wherein the protonation of the olefin by the solvated proton is the rate determining step. Under these conditions, the alcohol, which is in excess in the pores, breaks up the network of hydrogen-bonded 4QI-I groups, dissociating and solvating the proton. The higher reactivity of n-butanol versus methanol is a reflection of the acidity order of the corresponding proton on the alcohol. That is, due to the lower basic@ of n-butanol, a more acidic and more active ROH$ catalytic species is produced. On increasing the olefin/alcohol ratio (a > 1.7) the rate increases as the methanol decreases, until a maximum in the rate is reached. A further reduction in methanol leads to zero order in the olefin and first order in the alcohol (a > 10). It is inferred that at this point a real concerted mechanism has been

Fig, 3.

MTBE Synthesis. ‘Q&al areas delimited by ratio of reactants several hypothesized mechanisms [15].

at

reached wherein the isobutene is co-ordinated to the associated -SO#I groups and the interaction with the alcohols is the rate determining step. In addition, at ratios bigher than 3.5, the dimerization of isobutene also takes place. This was confirmed by I-Iech et al. [16]. It was concluded therefore that in the reaction of linear alcohols with tertiary olefins, catalyxed by ion-exchange resins, a transition in kinetic orders is observed which has been related to the ratios of the reactants and explained by mechanistic transitions. Gicquel and Torck [17] also examined the influence of methanol concentration on the activity of the Amberlyst 15 resin for the formation of MTBE, together with the effect of the polarity of the medium on the reaction thermodynamics. The synthesis and decomposition of MTBE were carried out in the temperature rage of 50 - 95 “C using a stainless steel double-jacketed reactor in batch-wise operation. The reaction medium was agitated at 575 rpm by a m~etic-Eve turbine. A ~~~-~helwood model rate expression was selected in order to interpret the results. Relative rate constants and equilibrium constants are given as a function of temperature and the activation energy and enthalpy of reaction were also determined. It was concluded by these authors as well that the resin catalysts

29

are sensitive to the concentration of methanol, with the reaction rates increasing sharply when the concentration is lowered. Furthermore, at a given temperature, the variation in the apparent equilibrium constant is mainly due to the variation of the methanol activity coefficient as a function of its molar fraction. The effect of temperature on the normalised etherification reaction rate of 100 % isobutene hydrocarbon feed for a methanol/isobutene feed ratio of 2 was also reported by Gulf Canada [12] and their results at high space velocity are shown in Table 1. In order to test whether the reaction was mass transfer or Table 1. Integral reactor data for MTBE synthesisa 1121. Space Velocnyb

Temperature “C

Reaction ROW

13.5 13.5 f3.5 13.5 13.5

62 80 89 100 107

1.38 1.91 2.6Q 3.60 3.62

31.2 44.1 58.9 76.6 79.1 Wydrocarbon feed is 100 % ICz;

t&OH/K,

= 2.0;

Selectivity:97 - 99 %.

bBased on IC;. cBased on dry catalyst.

diffusion rather than kinetically controlled or limited, a series of experiments were carried out by varying the Reynolds number of fiow through the reactor, while rn~t~~ the reciprocal space velocity constant and determining the effect on conversion. The observation that the conversion remained constant (Fig. 4) iudicates that mass transfer does not control the reaction. Another indication that chemical kinetics and not mass transfer is rate limiting, is that identical conversion and reaction rates were obtained with two different catalyst particle sizes (+ 16, and + 48 to -35 mesh). The results shown in Fig. 5, i.e. identical conversions and reaction rates were obtained for two different catalyst particle sines, provide further support that the reaction is not mass transfer controlled. Another evaluation of Al5 as a catalyst for MTBE synthesis under non-isothermal conditions believed to be compatible with those in industry was reported by Voloch et al. [18]. The study was carried out iu a bench-scale, plug-flow reactor system. Variables considered included temperature, flow rate and feed composition. An integrated rate expression was used to estimate kinetic constants at 26, 51 and 67 “C using data obtained from the plug-flow reactor.

30

25-

I

0 0

I 20

I Modified

I 40 Reynolds

I

I 60

I E

number

Fig. 4. Check for mass transfer limitations at 104 “C with a reciprocal space velocity based on isobutene of 0.45 [12].

04

I 160

I

I

I

160 Temperature

I

200 *F

I

1

220

F’ig. 5. Effect of temperature and catalyst particle size (0 35-48 Mesh, 0 -I-16 Mesh) on the % conversion to MTBE [l2]. Conditions which maximise selectivity and productivity, together with other catalyst characteristics, were discussed briefly. Colombo et al. [19] evaluated the equilibrium constant of the reaction in the liquid phase by means of predictive methods, using available literature values of the standard free energy of formation for the species involved and the UNIFAC estimates of activity coefficients to describe the liquid-phase non-ideality. Some

31

experimental values of the equilibrimn concentrations were obtained in a micro-pilot plant and the predicted equilibrium constants were found to agree quite well with the experimental values. In their 1987 report [20], Ancillotti et al. described the modified Snamprogetti process for the synthesis of MTBE from butadiene-rich steam cracking C4 cuts, As the authors point out, some problems associated with the presence of butadiene in the feed streams are undesirable reactions such as peroxidation and pol~e~tio~ and also the reaction between methanol and butadiene to produce butenyl ethers. The reaction rates for the addition of methanol to the following three olefinic substrates in units of l/(acid equiv.)(sec) at 80 “C are: isobutene

1 x 1o-2

butadiene

2x lo6

cis-2-butene

2x10”

and show that the butadiene reaction rate is greater than that for the linear butene but less thau that for isobutene. This order of rates is reSected in the relative carbocation stability according to the two-step reaction mechanism where the olefin protonation is regarded as the first and rate cornroIling step: CH3 CJ&-C/H3

r H-CH-=CH=-CH-CH3 gt

> CH3-CH-CH-CH3 G3 l!I

Tire butenyl ethers identified in the reaction product were essentially 3-method-l-butene and l-melon-~butene obtained from l-2 and l-4 methanol addition respectively. The reactivity difference between isobutene and butadiene is large enough to allow isobutene to react selectively, but only in a narrow area of operating conditions [21]. The use of contact times exceeding the minimum value required to reach the equilibrium in MTBE synthesis led to an increased butenyl ether formation and to MTBE decomposition due to the competitive reaction of metbauol with butadiene. The typical composition of processed C4 fractions is indicated iu Table 2 and a detailed analysis of the product is provided in Table 3. The h4TBE stream shows the presence of very low amounts of astir, such as the above-mentioned butenyl ethers, along with minor amounts of ter&butyl alcohol, methanol, 2-methoxy-butane and dimer. Rebfinger and Hoffmanu [22] recently reported on the determination of the micrakinetics for the liquid-phase synthesis of MTBE using Al5 and a specially prepared resin, CVT, which had a lower degree of cross-linking (7.5 % DVB). The experiments were conducted in a continuously stirred-tank reactor (106 ml,

32

Table 2.

Composttlon of processed C4 stream [20]. Wt%

es’s n- butane 1-butene lsobutyiene

fnvlsl-butene cis-2-butene 1,3-butadlene 1,2-butadlene propane vlnylacetylene 1-butyne CR’S

Table 3.

0.04 5.12 1.60 14.50 22.63 6.16 3.66 46.74 0.22 (32 ppm) (360 ppm) (210 ppm) 0.10

Composition of MTBE stream [20]. Wt%

MTBE methanol ten-butylalcohol 3-methoxy-1-butene 1-rn~-2-b~~e 2-methane heavy products

99.10 0.01 0.20 0.47 0.18 0.01 0.02

water peroxvciationinhibitor

stainless steel) in the temperature range of 50 - 90 “C. The temperature was controlled within 0.2 “C by immersing the whole reactor in a thermostated water bath [LB]. The experimental results were described by a three-parameter model based on a Langmuir-Hinshelwood rate expression in liquid phase activities from the UNIQUAC method. The developed microkinetic model was found to be independent of the degree of cross-linking of the resin in the investigated range of 7.5 - 20 % DVB by weight. The nature of the C4 solvent, n-olefin or n-alkane, does not infiuence the intrinsic rate of MTBE synthesis. Also, no pressure dependence could be observed up to 21 atm. In the second part of their study [24], they examined the influence of the macropore diffusion of methanol on the activity and selectivity of the resin catalyst. In the limiting case of methanol macropore diffusion as the rate condos step, the results were in agreement with a shell-core model, according to which almost none of the

33

me~~ol-~n~ core produces the by-product diisobutene, while only the outer shell forms the main product MTBE. defend effectiveness factors, q, for the isothermal catalyst beads were determined to be up to q = 5, while the corresponding theoretically calculated value of 7 was two-thirds of this value. In order to gain a better insight into the formation of the dimer, diisobutene, Rebf?nger and Hoffmann [23] conducted additional experiments, studying this reaction on its own and not as a side reaction of the MTBE synthesis. The formation of the dimer was studied in the temperature range of 60 - 90 “c using A15 as catalyst_ The reaction showed no steady-state behaviour and the catalytic activity declined at a rate dependent on the reaction conditions. Time constants for the activity loss were determined in the range 3.5 - 3.0 h. It was also observed that the deactivated Al5 catalyst from the diisobutene formation experiments could be regenerated through subjection to the WE synthesis experiment. The deactivation was assumed to be caused by blockage of the microparticle gel phase by higher isobutene oligomers. The fo~tion of ~obutene, using l-butene as solvent, was second order with respect to isobutene and showed an apparent activation energy of about 40 kJ/mol. The vapour-phase addition of methanol to isobutene over an Amberlyst 15 catalyst was recently studied by Tejero et al. [10,25]. The equilibrium constant for the reaction was determined experimentally in a continuous-flow reactor at atmospheric pressure and in the temperature range of 40 - 110 “C! [WI. When a mixture in eq~b~~ was fed into the reactor, the temperature of the catalyst bed did not change. Given that the hFI”BE synthesis reaction is fairly exothermic, this was considered a rough indication that the composition of the mixture did correspond to the equilibrium composition at the particular temperature. This method was found to be suitable and the equilibrium constants obtained were found to agree satisfactorily with the predicted values calculated from literature data. In addition, the values of AHo, AS0 and AGo at 298 K, deduced from the variation of the eq~ib~um constant with temperature in the range 40 - 110 “C, agreed with the values calculated from the literature data, within tbe limits of experimental error. Rate data were also obtained at 41 - 61.5 “C [lo]. The best-fitting Langmuir-Hinshelwood-Watson rate equation was derived from a mechanism whose rate-determinin g step is the reaction between the methanol adsorbed molecularly on one centre and the isobutene adsorbed on two centres. The authors also concluded that the proposed mechanism is ~e~o~~y consistent.

34

3.

THE USE OF ALTERNATIVEINORGANICACIDS

As noted in the previous section, the current industrial process for the production of MTBE utilises acidic cation-exchange resins as catalysts. The information available on alternative catalyst systems is somewhat scant and mainly involves recent research concerning zeolites. Early work identified that MTBE could be synthesized from methanol and tert-butyl alcohol or isobutene in the presence of mineral’ acids, such as sulphuric acid [26,2q; however, such a method was not particularly selective due to by-product fo~atio~ from dehydration reactions. More recentIy, Bitar et al. [28] of ARC0 have reviewed the features that should be considered in the design of an optimized process and hence these features are relevant to the design of an improved catalyst system 1.

Temperature: this is required to be as low as possible to maximize the equilibrium isobutene conversion and to minimize by-product formation.

2.

Me~~o~isobut~e ratio: this should be as near stoicbiometric as possible to reduce the cost of recovery and recycle of the unreacted methanol.

3.

Isobutene conversion: for single stage operation with acidic ion-exchange resins, 90 - 96 % conversion of isobutene is readily achievable; conversions above this level require the operation of an additional conversion stage, Clearly, novel catalysts need to match or better these criteria.

4.

Catalyst lifetime: operation at low temperature with current exchange resin catalysts permits lifetimes up to two years to be achieved and this must therefore serve as a benchmark for the design of improved catalysts.

With respect to catalyst lifetime for heterogeneous catalysts, it is well-known that for acid-catalyzed reactions, zeolites, particularly the pentasil zeolite ZSM-5, can demonstrate long operational life, e.g. as demonstrated by H-ZSM-5 for the methanol-to-gasoline process [29]. It is therefore not surprising that zeolites have been studied as catalysts for MTESE production. Chu and Kuhl [30] of Mobil Research and Development Corporation, studied zeolites @, ZSM-5, ZSM-11, rare-earth-exchanged zeolite Y and mordenite as catalysts for methanol and isobutene conversion in the vapour phase and their results are given in Table 4. This work was carried out in the vapour phase so that the selectivity of the various zeolites could be easily distinguished. From this preliminary work, it was concluded that the related medium-pore zeolites ZSM-5 and ZSM-11 gave the highest isobutene conversions and MTBE selectivities. Small-pore zeolites, e.g. synthetic ferrierite, are inactive for this reaction, since isobutene cannot enter the zeolitic cage structure. Chu and Kuhl rationalised the

Formation of MTBE over zeollte catalysts in the vapour phase [30].

Table 4. ZeOlW

Mtienite

ZSMS (70)

ZSM-11 (m)

Beta (a)

REHY (63)b

REAIY (5.1)0

1.07 1.14 82 93

1.46 1.53 8293

1.20 1.18 8293

1.06 1.04 82 93

1.04 1.05 8293

0.98 0.98 8293

8.4 7.1 6.0 14.1 58.3 33.5

13.9 9.1 23.0 27.3 37.7 25.0

11.3 10.6 0.47 1.9 96.0 85.0

25.3 23.4 0.3 1.3 98.8 94.9

30.5 25.3 0 0.1 100 99.6

25.f 21.0 0.1 0.2 99.4 99.0

(26)

Reaction conditionsd molar ratio0 tempemture/"C C~~~on~~~s to MTBE I% tOc8ddkI/% 80ieCt~

t0

MTBE/%

*Figure in brackets gives SiO&120s ratio. bREHY prepared by ion-exchange of calclnecl RENaY with (NH&SO,,, RE = rare earth. cREAY prepared by ion-exchange of calcined RENaY with Ai2(S04)B. dWHSV = 3.4 h-1,totalw&M of methanol and lsobutene per g catalyst. *Molar ratio methanoilisobutene.

relative reactivity of the zeolites in terms of the difference in diffusion rate of the reactants within the zeolitic channels. For the medium-pore zeolites ZSM-5 and ZSM-11, methanol diffuses more rapidly than isobutene and hence an isobutene molecule migrating within the zeolitic pore system encounters a high excess of adsorbed methanol and reacts to form MIBE in high selectivity. The lower selectivities observed for mordenite and zeolite /I were in line with the lack of shape selectivity in the diffusion of methanol and isobutene in the large-pore zeolites. The high selectivity observed for rare-earth-exchanged zeolite Y was considered to be due to the preferential a~o~~on of methanol by the more polar zeolite of lower SiO&4&03 ratio. One similarity between the large-pore zeolites tested @, mordenite, Y) and the commercial Amberlyst 15 resin catalyst was that the selectivity of MTBE increased with temperature, whereas this was not observed for ZSM-5 and ZSM-11. Chu and Kuhl PO] compared ZSM-5 and Z-SM-11 with the commercial A15 catalyst under liquid-phase conditions more similar to those used in the commercial process (Table 5). ZSM-5 was found to give superior selectivity to MTBE over the broad range of me~~o~isobutene ratios ~v~tigated when compared to Amberlyst 15, for which the selectivity approached that of the zeolite only at high ratios (Fig. 6). Chu and K&l, recognising this study to be an example of a low temperature application of zeolite catalysts, concluded that xeolites ZSM-5 and ZSM-11 offered a number of distinct advantages over the A15 catalyst, namely:

36

1.

bigb thermal stability and no acid efnuent;

2.

high selectivity to MTB&

3.

less sensitivity to methanol/isobutene

4.

high MTBE output due to operation at high temperature vel&y;

5.

ease of reactivation by standard calcination procedures.

ratio; and high space

Table 5. Comparison of ZSM-5,ZSM-11 and Amberlyst 15 In the liquid phase [30]. ZSM-11

zsM-5

Ze0lite

molar ratioa

1.00 1.05

1.10

1.05 1.10

WHsvih-'

1.75 1.71 108 108

1.68 108

1.49 1.41 8690

c4H8converslon/%

89.6 89.0

MTBE yiddb/% MTBE selectMty/%

59.6 89.8 loo loo

90.1 90.1

88.7 90.2 86.7 90.2

loo

loo loo

peak temperature/T

Amberiyst 15 1.00 1.05 1.03 1.01 77 70

1.10 1.01 62

93.1 93.4 86.0 89.4 92.4 95.7

94.5 92.7 98.1

amethand/Isobutene mdar ratio. b% of theoretical yield.

0.90

Fig. 6.

Ix) CH,CWi-C,,H,

1.1 Wr

ratlo

Liquid phase selectivity to MTJSEas a function of methanol/isobutene ratio [30].

37

Subsequent patents to Mobil Oil Corporation [31,32] disclosed the use of zeolite catalysts in a combined MTO/etherification process for the conversion of methanol to MTBE. In a detailed study, Tau and Davis [33] also carried out a comparative study of a range of solid acid catalysts for the conversion of ethanol and isobutene to form ETBE, i.e. the analogous ethanol reaction to mE synthesis. Six catalysts were selected for study: Amberlyst 15, Amberlyst 35, ZSM-5 (SiO$Al& ratio = 60), Union Carbide zeolite catalyst S-115 ~Si~~2~ ratio = Sot), a opposed fluorocarbonsulphuric acid polymer catalyst (FCSA) that had previously been noted as a very strong acid [34], and phenylphosphoric acid resin (PPA) supported on active carbon. Tau and Davis tested all six catalysts under a wide range of conditions and the optimal data are given in Table 6 for vapour-phase studies at Table 6. Comparison of the formation of ETBE over diffwent catalysts at atmospheric pressure [33f. -w@ temperaturePCB If&H8 conversion/% ETBE 8electivity/%b cs Sdwltivity/%b ethyl ether selectivity/%’ tert-butyialcohol 8electivity/%b

Alii 60 43.5 99.6 0.3 CO.1
A35

ZSM-s S-Ill

69 39.3 98.9 1.0 co.2 co.1

PPA

FCSA

120 9.6 100

146 es i loo

200 s 0.5 loo

60 21.3 loo

-

-

-

-

0.6

*wHsv = 3.4-l, ethanolkobutene molar ratio = 1.0. bbased on isobutene. chased on ethanol.

atmospheric pressure. S-115 and PPA were not found to be particttlarly active for EXBE synthesis. For the other four catalysts, the isobutene conversion was observed to go through a maximum with increasing temperature (Fig. 7). Tau and Davis further tested catalysts A15, A35, FCSA and ZSM-5 in a liquid-phase reactor at total pressure of 200 and 300 psi (Table 7). Increasing the pressure from 200 psi to 300 psi did not lead to a significant increase in the isobutene conversion. In this study, Tau and Davis noted that the ~nve~ion for ZSM-5 is considerably lower than that of the commercial Al5 catalysq this is an observation in contrast to the study of Chu and Kuhl [30] (compare Tables 5 and 7). However, these studies are not directly comparable, as Tau and Davis utilised a much higher temperature for the ZSM-5 experiment compared to Chu and Kuhl, and this will limit the equilibrium isobutene conversion that can be

38

IO-

_)(--)

,_‘)y”X”

r I

I

40

t

60

t

I

1

60 100 Temperature OC

I

t

120

(

J

Ia4(3

Fig. 7. The conversion of isobutene over dii&ent acid catalysts as a tunction of temperature at atmospheric pressure: 0 Al5, q A35, oFCSA, x ZSM-5 [33]. Table 7. Pressure

ETBE formation in the liquid phase for different catalysts [33]. 200

(psi)

300

catal ys?

Al5

A35

ZSM-5

FCSA

Al5

A35

ZSM-5

FCSA

temperaturePC IC4Hsconversio~ ETBE cs sektMty/% selectivit

%$60 86.1 98.8 1.2

60 85.8 98.4 1.4

125 56.6 99.9

80 72.5 989.8 0.2

60 88.9 98.4 1.6

60 88.3 97.6 2.2

135 61.6 99.5 0.2

80 80.4 99.6 0.4

*WHSV = 3.0 h-1, ethanol/lsobutene bbased on Isobutene.

-

molar ratio = 1.0.

achieved. The high selectivity to the ether, noted by Chu and Kuhl [30] for the zeolite catalyst, was confirmed in the study of Tau and Davis. One important result obtained from the study of Tau and Davis was the observation that the FCSA catalyst exhibited similar activity to that of the A15 catalyst. However, FCSA exhibits a higher selectivity, particularly for ethanol/isobutene ratios of less than unity. The selectivity difference was typically 1 - 2 % and, in addition, the FCSA appeared to be much less sensitive to changes in the ethanol/isobutene ratio. The higher temperature stability of FCSA compared to Amberlyst 15 was also demonstrated by Tau and Davis, who successfully operated the FCSA catalyst at temperatures up to 130 “C without detecting acidity in the reactor effluent. On the basis of the studies of Chu and

Kuhl [30] and Tau and Davis, it can be concluded that the pentasil zeolite ZSM-5 and the fluorocarbonsulphuric acid polymer catalyst offer considerable scope for the design of improved catalysts for h0’BE production. More recently, patents to the Mobil Oil Corporation [35,36] have disclosed the use of zeolite @ for the synthesis of ethers from the reaction of alcohols and iso-olefins. This is of interest, since the original study of Chu and Kuhl [30] (Table 4) indicates that the large pore zeolite /I is not particularly selective for this reaction. Haag and Bell [35] compared the use of zeolite /I with the commercial catalyst Al5 for the synthesis of set-amyl methyl ether (SAME) from methanol and isopentene, and showed that the former was more active (Table 8). In this experiment zeolite t9 was diluted with silica sand and used in extrudate form. The enhanced selectivity to this bulky ether may be due to the larger pore size of zeolite j?. Haag and Bell also demonstrated the use of zeolite /I as a catalyst for the synthesis of methyl isopropyl ether (MIPE) from the reaction of propene and methanol at 160 - 163 ‘C and WHSV 0.12 - 3.70 h-1. Again very high selectivities to the ether were reported. Subsequently, Harandi and Owen [36] disclosed the use of zeolite #I as an etherification catalyst in a two-stage process for which the first stage involves the isomer&&on of an olefin-containing feedstock over a silicotitanate catalyst. Based on these disclosures, it is clear that zeolite /3 also warrants further attention as a potential future commercial catalyst for MTBE synthesis. Table 8. Conversion of methanol and P-pentene to SAME [35]. Catalyst time on stream/h WHSV/h-‘a 2-C&H10conversion/% Reaction effluent/mass %: water methanol dimethyiether 2 pentene SAME

Zeolite B 4

4.03 15.2

Amberlyat 15 5 16.36 5.0

0.9

0.8

23.6 2.0

31.9 0.3 62.9 3.1 1.0

5 3.93 6.3

3.6 9.7 8.4 71.8 6.0 0.6

7 3.93 8.6

0.4 6.2 13.9 68.7 8.3 0.6

aTemperature 150 - 152 “c, pressure 950 PSI.

A comparative study has been carried out on the use of three different acid catalysts, viz. the Al5 resin and zeolites Y and ZSM-5, as well as of the triflic acid-coated counterparts of these zeolites for the gas-phase synthesis of MTBE

[37]. In the case of ZSM-5, its lower acidity cannot be adjusted by loading with tritlic acid because of the negative influence of the coating on the diffusion of reactant and product molecules. Triflic acid-loaded zeolite Y, on the other hand, appears to be as active as Al5 in the gas phase synthesis of MTBE and also has a much lower production of by-products (see Table 9). Furthermore, it is more thermally stable than the commercial resin catalyst and could therefore be a valid alternative to the conventional catalyst. Table 9.

Maximum MTBE yield obtained with the triflic acid-loaded zeolites [37].

Catalyst

Amberly8t 15

Temperature “c MTBE yield (C-atom %) Ce yield

66 47.5 9.7

ZSM-5jTFA (3 wt %I 65 40.6 0.2

Y/lFA (3wt%) 65 46.1 0.4

Bylira et al. [40] and Adams et al. 1411 reported that ion-exchanged montmorillonites can catalyse the production of MTBE from methanol and isobutene or from methanol and tert-butyl alcohol [42]. Methods have also been described for the acid-treatment of montmorillonite-based catalysts and the addition of water to improve their catalytic activity for the reaction [43,44]. In a subsequent investigation by Adams et al. [45], montmorillonite-based catalysts were compared with the Al5 catalyst for the reaction of methanol and isobutene or TBA, When 1,4-dioxan was used as solvent, Al3+-exchanged montmorillonites had about half the efficiency of the resin Amberlyst 15 at 60 “C, they were, however, about twice as efficient at this temperature as Ti3+-montmorillonite or KlO, a commercially available acid-treated bentonite. Montmorillonite exchanged with Chlorhydrol solutions to give interlayer WI~WWD~%~’ + ions and pillared clays derived from such materials were poor catalysts, as was K306, a more drastically acid-treated bentonite-based commercial catalyst. Freeze-drying of the Als+ -exchanged clay before reaction of isobutene and methanol in dioxan was 44 kJ/mol for an Al3+-clay catalyst compared with 25 kJ/mol for reactions catalyzed by Amberlyst 15. With no solvent (as in industrial processes), the rates of reaction were considerably slower for both the clay- and resin-catalyzed reactions. As has been found previously for resin-catalyzed reactions using stoichiometric amounts or an excess of methanol, the rate was proportional to the isobutene concentration, and the rate determining step appeared to be the protonation of the akene. The performance of the

41

Al3+-clay catalyst was increased by reducing the water content of the clay, In most reactions the clay catalysts were equilibrated at 12 % relative humidity. EJxposure of the clay to a low vacuum (10-l torr) before use increased its catalytic activity from 50 to 60 % of that of Amberlyst 15.

The overall

considerations to be takeu iuto account for the design of the

process are: -

the exo~e~~~ of the forward reaction; the sensitivity of the resins to temperature in excess of about 90 - 95 Oc, the desire to use pressures required to maintaiu olefin feeds in the liquid state; the need to “debutauixe”, i.e. to remove unreactive Cq components, n-butenes, isobutaue and n-butane; - ovation of hKIBE usually accompanied by recycle of unreacted methanol.

4.1. HW/UOP [46] The process layout is depicted iu Fig. 8. The c4 stream typically cousists of isobuteue first purified to remove resin contammants before combiuiug witb methanol. Alternatively, FCC light gasoline may be fed and, in this case, tbe combined feed is reacted over the acid ion-exchange resin contaiued in two fixed-bed adiabatic reactors (2,3). If the reactive olefin concentration in the feed is high, as in the case of steam-cracker Cd’s or C4 dehy~oge~tion products, the first reactor (2) is either an isothermal rn~ti-~b~~ or adiabatic reactor. In the latter case, a small slipstream of the first reactor (2) effluent is cooled and recycled back to the inlet of that reactor. Improved temperature ,control results, and this leads to improved yield and resin life. Carbon steel ~~~~on is consistent with the mild conditions required for the reaction. MTBE is recovered at a very high purity (99 % + ) as bottoms from the butene cohmm (4). The overhead contaius excess methanol and unreacted C4 hydrocarbons (rafEuate). The excess methanol is recovered with a simple water-wash system (5,6) and recycled to the inlet of the first reactor. Spent Ct hydrocarbons are usually sent to au alkylation unit.

uses a single fixed-bed tubular reactor and h4TBE is separated from C& residue iu a debutanixer (Fig. 1). The conversion of isobutene ranges from 60 to 80 % for dilute catalytic cracker streams and from 97 to 98 % for steam cracker C&z. Water-washiug of the C4 reduces residual methanol to below 10 ppm. This

42

Spent C4 hydrocarbons to alkyltitlon l

Cq Hydrocarbons

1 High Methanol

grade

MTBE

product .

I __

F’ig. 8. Hiils-UOP Process [461. 4.3. Atlantic Richfi~d [47] Either isobutene installing shifts the

a single-stage or two-stage reaction train is used, the former resulting in conversions of 95 %. The conversion may be raised above 99 % by a debutanizer between the two reactors. The intermediate debutanizer equilibrium toward forming more M’IBE.

4.4. CD Tech [48] and Ethermax Process [4] In the CD Tech Process C4 feed is mixed with fresh and recycle methanol, preheated and fed (Fig. 9) to a fixed-bed downflow adiabatic reactor (1) in which the liquid becomes heated to bubble point by the heat of reaction, and with small vapour generation eliminates the need for external cooling. After reaction, the mixture flows from the primary reactor to a catalytic distillation reaction column (2) in which further reaction and distillation proceed simultaneously. A reduction in total installed cost results, with the MTBE being removed from the reaction

43

C4’s

Recycle

Methanol

rafflnate

I I

Methanol

1'

LA

.

t

-

---dr Water

MTBE

Fig. 9. MTBE Synthesis (CD Tech) [48]. zone by distillation as it forms. The reaction is thereby permitted to proceed beyond the thermodynamic limit and higher conversions (99 % +) are achieved. This type of approach is also a feature of the Ethermax Process recently announced by UOP [4]. Useful for MTBE production, it becomes even more attractive for ETBE and TAME production where equilibrium constraints are higher. For example, isoamylene conversion can be increased from 70 % to 91 %. 4.5. Phillips Petroleum Company [49] The unique feature of this process consists of the use of a methanol fractionator which ensures very high recovery. All unreacted methanol is fractionated away from the MTBE bottoms, saving substantial steam and the residual butene stream may be sent directly without further treatment to an alkylation or polymerization unit. 4.6. BP [!50] A key feature of BP’s technology (Fig. 10) is a novel trifunctional catalyst which performs three reactions simultaneously: etherification of iso-oletins, selective hydrogenation of unwanted diolefins and hydroisomerization of oleCns by means of double-bond-shift.

44

Hydrogen

I

1

t

1 I 2

C4/% Feed

Methanol

b

c4 to alkylatlon

I

3

Recycle

Mixed Ethers t

Methanol

4

Fig. 10. Etherol Process

(BP) [48].

In the C& option, feed and methanol are purified (1) and, after the addition of a small quantity of hydrogen, pass to two fixed-bed reactors (2) and (3) containing an acid resin catalyst. The reactors operate liquid-filled and some recycle of cooled first-reactor effluent occurs in order to fine control temperature to increase yield and extend catalyst life. Debutanizing is effected in (4) while methanol is recovered in (5) and recycled to the guard vessel (1). 4.7.

Other developments

A series of processing steps involving the close integration of the ether&&on reaction with those of isomer&&ion, dehydrogenation and methanol synthesis have been outlined by Davy International and Davy McKee [49]. The isomer&&ion of n-butane (obtained from an LPG fractionation plant) is effected over, for example, a platinum catalyst at 150 - 200 “C and 15 - 30 bar. After distillation, the isobutane is dehydrogenated (using, for example, Catofin technology developed by Air Products and Chemicals and now owned by United Catalysts with licensing through ABB Lummus Crest) and the C4 stream passed to the etherification stage. In an integrated complex, a common steam system can be employed with optimization of the steam levels for high pressure use in turbines, medium

45

pressure use in the conversion process and low pressure use in steam boilers. ~~~ utilizationof off-gas streams for use as fuel or feedstock enhances energy efficiency and product yield. Finally, a recent development announced by Mobil [50], involving the Mobil Olefins to Gasoline (MOG) process, is claimed to help reduce the capital costs of a new MJI’BE plant by some 30 %. The MOG process leads to high octane gasoline from light olefins (including ethylene) contained in low value refinery streams. The gasoline is produced in a fluidized-bed reactor containing zeolite ZSM-5 catalyst. Oligomerization, alkylation and skeletal isomerization occur simultaneously. Integrating the MOG process with MTBE production is made possible by the fact that MOG can essentially be operated by feeding methanol directly, when the well-known methanol to olefins reaction occurs (Fig. 11). There is then no need for a metbanol recovery unit to be installed as is normally the case. To Fuel

C$C3 Feed

t __.__,

MeOH Feed c, Feed J

1st stage MTBE Reac to

t1

unreacted Oleflns with Methanol

MOG UNIT

1

2nd Stage MTBE

Fig. 11. MOG/MTBE Process (Mobil-Badger) [SO].

LPG b

c

MOG Product

-

*1-rnr .._.._,or

4.8. MTBE cracking As a footnote to this review of MTBE production, it is fitting to remark that the reverse process of MTBE cracking has also received attention, because it provides a convenient route to high purity isobutene for use in the manufactnre of elastomers (butyl rubber and polyisobutene) and other chemicals (tert-butyl derivatives, diisobutene, polybutenes, etc.). Fattore et al. [Sl] discuss the advantages of recovering isobutene from MTBE, these lie in the absence of environmental and corrosion problems associated with the conventionally used systems (e.g. extraction with sulphuric acid) in the high purity of the olefin obtained and in the decoupling of isobutene production from that of propylene oxide synthesis. For the reverse reaction, inorganic solid acids would be used since the temperatures used (in the range 160 - 220 “C) are significantly higher than those associated with stable operation of the ion-exchange resins. A specially treated y-ahunina [52] appears to offer excellent performance. Problems which could be encountered include dehydration of product methanol to dimethylether and some isobutene loss by reaction with the water so formed (leading to TBA). 5.

CONCLUSIONS

Despite their success in providing the basis of commercial processes for the production of MTBE and related ethers, sulphonated ion-exchange resins are being challenged by alternative catalyst systems. Zeolites ZSM-5 and ZSM-11 appear to be capable of superior performance, particularly with regard to lower sensitivity of synthesis to alcohoVolefin ratios and higher outputs. In addition, supported fluorocarbonsulphuric acid polymer catalyst (FCSA) also has appealing properties and appears to exhibit higher thermal stability than, for example, Amberlyst 15. Because MTBE is a relatively low-value chemical, the economics of production are of critical importance, vis B vis its application as a fuel component. Considerable attention has therefore been paid to designing high-efficiency commercial processes and the integration of product distillation and catalytic etherification is a good example of this. Closer integration of etherification with other refinery operations (olefin production, methanol synthesis) has been sought and it is likely that further development in this direction will continue. In addition, it seems likely that we will see further efforts aimed at the production of mixtures of fuel ethers from suitable mixed feeds (hydrocarbons and/or alcohols) where these are available. The synthesis of fuel ethers presents the classic challenge to catalytic technologists: a strongly exothermic reaction proceeding at comparatively low temperature with catalysts having a limited thermal stability. The challenge is being

taken up and severaI novel lines of ~v~~atio~ anticipated.

some already just ears

are

6.

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