MTBE process

The MTBE reactive distillation process was patented several decades ago, and the process was widely used in the petroleum industry. Many reactive columns were installed around the world to produce MTBE, which was blended into gasoline. This process was probably the largest application of reactive distillation in terms of the number of columns and total production capacity. Because MTBE presents groundwater contamination problems, it is gradually being phased out of use in gasoline. 

The reactive distillation column is essentially a ternary system with inerts. The liquid-phase reversible reaction is 

Reactive Distillation Design and Control. By William L. Luyben and Cheng-Ching Yu Copyright 2008 John Wiley Sons, Inc. 

DESIGN OF MTBE AND ETBE REACTIVE DISTILLATION COLUMNS 

The heavy component is MTBE, which leaves the reactive distillation column in the bottoms. The isobutene feed is contained in a mixed C4 stream fiom an upstream refinery unit. This stream contains a number of other C4 hydrocarbons because of the difficulty of separating the various components with very similar relative volatilities. In the numerical example, we assume that n-butene is the chemically inert component. Most of this inert component leaves in the distillate stream. 

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The H2SO4 catalyst produces a high octane product of similar composition from either 2-butene or 1-butene. This fact suggests that the isomerization of 1-butene to 2-butene is more complete than in the HF system. Isobutylene produces a slightly lower product octane than do the / -butenes. The location of a methyl tert-huty ether [1634-04-4] (MTBE) process upstream of the H2SO4 alkylation unit has a favorable effect on performance because isobutylene is selectively removed from the alkylation feed. 

This increase in isobutylene yield increases feed for downstream MTBE processing. 

Fig. 10.31. Two-stage MTBE process. Bitar, L S., Hazbun, E. A. and Piel, IV. J., Hydrocarbons Processing, 63, no. 10, 54. 1984, October. Copyright Gulf Publishing Company and reproduced by permission of the copyright owner.)...

Should MTBE be banned, what would be the logical replacement(s) There are several options available. Several refiners opted to build MTBE capacity and avoid purchasing the ether on the open market. MTBE units were an option to use the facility s isobutylenes. Several licensed processes can be used to convert existing MTBE units. Kvaerner and Lyondell Chemical Co. offer technologies to convert an MTBE unit to produce iso-octane, as shown in Fig. 18.27.12 Snamprogetti SpA and CDTECH also have an iso-octene/iso-octane process. These processes can use various feedstocks such as pure iso-butane, steam-cracked C4 raffinate, 50/50 iso-butane/iso-butene feeds, and FCC butane-butane streams. The process selectively dimerizes C4 olefins to iso-octene and then hydrogenates the iso-octene (di-iso-butene) into iso-octane. The processes were developed to provide an alternative to MTBE. The dimerization reactor uses a catalyst similar to that for MTBE processes thus, the MTBE reactor can easily be converted to... 

The catalysts used industrially in the MTBE process are sulphonated polystyrene resins of the macroreticular type. These strongly acidic materials are prepared by copolymerizing styrene and p-divinylbenzene in the presence of an organic compound that is a good solvent for the monomers but a poor swelling... 

Fig. 2.25. Flow scheme of the MTBE process with catalytic distillation.

Application The Uhde (Edeleanu) MTBE process combines methanol and isobutene to produce the high-octane oxygenate—methyl tertiary butyl ether (MTBE). 

Commercial plants The Uhde (Edeleanu) proprietary MTBE process has been successfully applied in five refineries. The accumulated licensed capacity exceeds 1 MMtpy. 

Floris, T., Peed, Oriani, G. Snaffl Progeedr Ante MTBE process benefits unleaded gasoline refiners. NFRA... 

One possible starting material for the production of Cio alcohols is the above-mentioned Raffinate-2, a C4 feedstock derived from mixed C4 streams of steam crackers. After butadiene has been removed from the mixed stream, Raffinate-1 is obtained. The isobutene content of Raffinate-1 is removed by conversion to MTBE (methyl t-butyl ether), leaving behind a stream rich in mixed butenes which do not react in the MTBE process this is designated Raffinate-2. Accordingly, in the USA and western Europe MTBE plants are the main consumers for Raffinate-2. 

The second stage in the process is required because the MTBE formation is an equilibrium reaction. The temperature needed ( 100°C) to achieve a sufficiently high rate of conversion means a decrease in isobutene equilibrium conversion (XiB = 0.9 at 65°C, Xjb = -0.75 at 100°C). The main side reaction in the MTBE process is the dimerization of isobutene towards di-isobutene (two isomers). Side reactions with essentially no significance are the formation of f-butyl alcohol (due to the presence of water as feed impurity), the formation of dimethyl ether from methyl alcohol, and the oligomerization of isobutene towards tri- and tetramers. A (three stage) process is also in operation which tolerates butadiene. The butadiene/ methyl alcohol reaction is faster than that of the n-butenes but consider-... 

Institut Frangais du Petrole (IFP) has developed a process, where pervaporation of methanol is used to debottleneck MTBE production. In the debutanizer columns used in MTBE processing, the MTBE/ methanol azeotrope results in a concentration of methanol at a point midway between the feed tray and the... 

The first CD packing used in the CD process for MTBE process consisted of an acidic ion-exchange resin such as Amberlyst 15. These macroporous resin beads are packed into fiberglass bags wrapped with demister wire, which are subsequently arranged into bales of Texas Tea Bags that are inserted into a certain location in the CD column. 

Figure 3.29 MTBE process by Valero Refining Marketing. Source [51].

Figure 3.33 shows a schematic of the CDTech MTBE/ETBE/TAME process. This is essentially the same as the CDTech MTBE process presented earlier. The process is unique in the sense of using a boiling point reactor and catalytic distillation (CD) [61]. The C4 feed and methanol is fed to the boiling point reactor (1). This is a fixed-bed downflow adiabatic reactor, in which the liquid is heated to its boiling point by the heat of reaction and... 

Feed Type C4 Effluent from a MTBE process ... 

Baur et al. [17] have compared the EQ and the NEQ models for the MTBE process. They underlined some counter-intuitive features of RD processes. For example, for a methanol feed location yielding a low-conversion steady state, the introduction of mass-transfer resistance (i. e., use of the NEQ model), leads to a conversion higher than that predicted by the EQ model). The introduction of a mass-transfer resistance alleviates a bad situation and has the effect of improving conversion. 

Sundmacher et al. [20] used both EQ stage (with Murphree efficiency) and NEQ models to simulate the MTBE and TAME processes. The reactions were handled using both quasi-homogeneous and heterogeneous methods. Simulation results were compared to experimental data obtained in two laboratory-scale columns. A detailed NEQ model was needed to describe the TAME process, but both NEQ and the EQ stage (with an efficiency of 0.8) model could adequately represent the MTBE process. 

Mohl et al. [21, 22] implemented a dynamic EQ model (with Murphree type efficiencies) in the DIVA simulator and carried out a numerical bifurcation and stability analysis on the MTBE and TAME processes. They also show that the window of opportunity for MSS to actually occur in the MTBE process is quite small. 

For the TAME process MSS occur in the kinetic regime and vanish when chemical equilibrium prevails. The window of opportunity for MSS in the TAME process is larger than for the MTBE process. 

Experimental confirmation of MSS in RD was provided by Thiel et al. [23] and by Rapmund et al. [24]. Mohl et al. [1] used a pilot scale column to produce MTBE and TAME. MSS were found experimentally when the column was used to produce TAME, but not in the MTBE process. The measured steady state temperature profiles for the low and high steady states for the TAME process are shown in... 

First simulation results on steady state multiplicity of etherification processes were obtained for the MTBE process by Jacobs and Krishna [45] and Nijhuis et al. [78]. These findings attracted considerable interest and triggered further research by others (e. g., [36, 80, 93]). In these papers, a column pressure of 11 bar has been considered, where the process is close to chemical equilibrium. Further, transport processes between vapor, liquid, and catalyst phase as well as transport processes inside the porous catalyst were neglected in a first step. Consequently, the multiplicity is caused by the special properties of the simultaneous phase and reaction equilibrium in such a system and can therefore be explained by means of reactive residue curve maps using oo/< -analysis [34, 35]. A similar type of multiplicity can occur in non-reactive azeotropic distillation [8]. 

However, it was shown that for the above conditions the multiplicity regions in the space of the adjustable operating parameters are fairly small for the MTBE process ]73]. This is illustrated in Fig. 10.13 for the pilot plant column treated in ]72, 73]. The bifurcation parameters are the heating rate Q and the reflux ratio R, which can be directly adjusted at the real plant. The parameter range is further decreased if a finite mass transfer between the vapor and the hquid phase is taken into account as shown in ]5, 40] for the column configuration of Jacobs and Krishna ]45]. Moreover, the multiplicity regions even seem to disappear entirely, when finite transport processes are taken additionally into account inside the catalyst [39]. Hence, practical relevance seems to be low. 

Fig. 10.13 MTBE process at p = 11 bar. Bifurcation diagram for different reflux ratios R (left), O denotes total reboil. Multiplicity region in the R/Q parameter plane (right)...

Despite the similar reaction mechanism, a completely different type of behavior was found for the TAME process [71-73]. This is due to the fact that the rate of reaction is one order of magnitude slower for TAME synthesis compared to MTBE synthesis. The behavior of the TAME process is illustrated in Fig. 10.14. In contrast to the MTBE process the TAME column is operated in the kinetic regime of the chemical reaction at a pressure of 2 bar. Under these conditions large parameter ranges with multiple steady states occur. The more detailed analysis by Mohl et al. [73] reveals that steady state multiplicity of the TAME process is caused by self-inhibition of the chemical reaction by the reactant methanol, which is adsorbed preferably on the catalyst surface. Steady state multiplicity is therefore caused by the nonlinear concentration dependence of the chemical reaction rate. Consequently, a similar type of behavior can be observed for an isothermal CSTR. This effect is further in-... 

Comparison between theoretical predictions and experimental results showed, that, in contrast to the MTBE process, transport processes inside the catalyst were negligible for the TAME process. This is again due to the fact that the microkinetic rate of reaction is one order of magnitude slower than in the MTBE case and is therefore dominating for the operating conditions considered in this study. This was confirmed more rigorously in a recent paper by Higler et al. [39]. 

In tray columns the first mechanism is dominant. This can lead to a large number of different steady state solutions for a given set of operating conditions. If N is the number of steady states (typically an odd number). Then (N + l)/2 of these steady states are stable. This can lead to complex multi-stable dynamic behavior during column startup and set-point or load changes. These phenomena were observed for vanishing as well as for finite intra-particle mass transfer resistance. An example with a total number of six trays (two reactive and two non-reactive trays plus reboiler and condenser) is shown in Fig. 10.16 for the well-known MTBE process. In contrast to the previous section, the column is now operated in the kinetic... 

In all cases, kinetic multiplicity can be avoided by an increase of the Damkohler number, that is an increase of the number of active sites on the catalyst, or a decrease of the feed rate. Moreover, multiplicity will vanish if the column pressure is increased. In all cases the column gets closer to chemical equilibrium. This is consistent with previous experimental studies for the MTBE process at 7 bar and low feed rates [103] where no multiplicity was found. 

For the MTBE process also oscillatory behavior was reported in the literature. Potential sources for such an oscillatory behavior are either unwanted periodic forcing (e. g., by badly tuned controllers), fluid dynamic instabilities, or instabilities of the concentration dynamics. 

Fluid dynamic instabilities were reported for the MTBE process by Sundmacher and Hoffmann [104]. The cycle times for fluid dynamic oscillations are typically in... 

Conventional process. The conventional route in the synthesis of MTBE is described in detail in Kirk-Othmer (1994) and Peters et al. (2000). For instance, the Hiils-MTBE process (figure B.l) operates with a given molar excess of methanol on... 

Figure B.l. Conventional route for the synthesis of MTBE two-stage Hiils -MTBE process. Legend ROf tubular reactor R02-R03 adiabatic reactors HX heat exchanger M mixer D divider SOf-S02 distillation towers (adapted from Peters et al. (2000)).

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