Isobutene Conversion

The catalyst used in this process is a cation-exchange resin and is available from several manufacturers. Isobutene conversions of 94% are typical for FCC feedstocks. Higher conversions are attainable when processing steam-cracker C4 cuts that contain isobutene concentrations of about 25%. 

Very high isobutene conversion, in excess of 99%, can be achieved... 

The catalytically promoted liquid-phase etherification of isobutene with an excess of methanol to produce MTBE has been carried out on a commercial scale in conventional fixed-bed reactors since the 1970s isobutene conversion is on the order of 90-97%. MTBE can be subsequently separated from the inerts and excess methanol by distillation— although this is complicated by the presence of minimum boiling azeotropes between MTBE and methanol and between isobutene and methanol. Unreacted isobutene is, however, difficult to separate from n-butanes and n-butenes because of their low relative... 

ETBE is produced by the liquid-phase addition of ethanol (EtOH) to isobutene (IB) in presence of an acid catalyst. Since the reaction is exothermic and limited by the chemical equilibrium, the reactor outlet temperature is maintained as low as allowed by the catalyst activity in order to maximize the isobutene conversion. Operating temperatures range between 40 and 80 °C. 

As finishing reactors, to complete the isobutene conversion, it is also possible to use, in the second stage, a catalytic distillation tower that combines reaction and fractionation in a single unit operation. 

In the case of refinery cuts from FCC units, having a relatively low isobutene concentration (Table 11.2), the plant layout is less sophisticated because it is sufficient to achieve 90-95% of isobutene conversion in this case the plant configuration is based on a single reaction stage with tubular and adiabatic reactors in series with intermediate cooling. 

For example, on Cu-Ph reaction starts at 290 K, with increasing temperature the amount of methacrolein formed rises, attains its maximum value at 355-365 K and then at 430 K it decreases. The maximum degree of isobutene conversion is 25-30 %. On Cu-Iml and Cu-Sup isobutene oxidation starts at 420-450 K (Figure 1). Partial oxidation products (methacrolein, acetaldehyde, acetone - 0.05 vol. %) are formed with maximal rate at 520-550 K. Above 570 K isobutene is oxidized to CO2 and H2O The similar results are obtained over Ti-containing catalysts. 

Reaction temperature. Lower temperatures enhance isobutene conversion, prolong catalyst life, decrease formation of byproducts like diisobutene and tert-butyl alcohol. Higher temperatures cause catalyst deterioration, favor decomposition of product, and reduce yield. 

Isobutene conversion Conversion of isobutene depends on the number of stages in the process. A single-stage reaction achieves conversions of 90-96%, whereas two-stage reactions (where product is selectively removed before the second stage) achieve 99%. Higher conversions facilitate the fractionation of pure l—butene from the process raffinate. 

CDTech uses catalytic distillation to convert isobutene and methanol to MTBE, where the simultaneous reaction and fractionation of MTBE reactants and products takes place [51], A block diagram of this process is shown in Figure 3.31. The C4 feed from catalytic crackers undergoes fractionation to extract deleterious nitrogen compounds. It is then mixed with methanol in a BP reactor where 90% of the equilibrium reaction takes place. The reactor effluent is fed to the catalytic distillation (CD) tower where an overall isobutene conversion of 97% is achieved. The catalyst used is a conventional ion-exchange resin. This process selectively removes MTBE from the product to overcome the chemical equilibrium limitation of the reversible reaction. The MTBE product stream is further fiactionated to remove pentanes, which are sent to gasoline blending, whereas the raffinate from the catalytic distillation tower is washed with water and then fractionated to recover the methanol. 

This is carried out in the presence of mixed oxides based chiefly on molybdenum, bismuth, tellurium, eta, in the vapor phase, around 350 to 450°C, at low pressure, with residence times of 1 to 5 s to enable once-through isobutene conversion higher than 95 per cent, n-butenes conversion of about 80 to 85 per cent, and methacrolein and butadiene molar yields of 75 to 80 per cent Developed in particular by Japan Synthetic Rubber, Mitsubishi Rayon, Nippon Kayaku, Nippon Zetm, Ube, etc., this type of process comprises the succession of the following sequences ... 

Figure3 Isobutene conversion over 2wt.% Keggin on Si02, 2wt.% Dawson on Si02,...

This scheme will provide a total isobutene conversion up to 95%. With the double stage scheme, it is possible to reach more than 99%. 

Very high isobutene conversion, in excess of 99%, can be achieved through a debutanizer column with structured packings containing additional catalyst. This reactive distillation technique is particularly suited when the raffinate-stream from the MTBE unit will be used to produce a high-purity butene-1 product. 

Description Depending on conversion and investment requirements, various options are available to reach isobutene conversion ranging from 85 wt% to 99 wt%. 

Fig. 9.5 Left side Pd-Ag membrane reactor isobutene conversion vs. feed space velocity, compared with equilibrium-limited and fixed-bed reactor (argon swept, T = 723 K, after [33]) right side carbon membrane reactor conversion, in the countercurrent sweep and vacuum modes, as a function of feed molar flows at 500°C also denoted are the conventional (non-membrane) reactor conversion and the simulated countercurrent sweep mode behavior (after [23])...

The feed is composed of two streams. The first stream is a hydrocarbon stream that contains 30 mol % isobutene and 70 mol % 1-butene. The second stream, consisting of pure methanol, is in 5 per cent molar excess of the reaction stoichiometry. The hydrocarbon feed rate is 1000 kg h . Both streams are at 30°C and 1500 kPa. The reactor inlet temperature should be controlled at 70°C. The reactor outlet temperature will be higher than the inlet, since the reaction is exothermic and a considerable amount of heat is released. This has the effect of limiting the conversion of isobutene in the reactor. The reactor product should be cooled to around 40°C so that a second reaction stage can increase the isobutene conversion to around 99 per cent. The reactor pressure drop is 140 kPa, and the pressure drops through the exchangers are 70 kPa. The exchanger volumes can be estimated at 0.1 m each. 

How does the isobutene conversion vary during disturbances ... 

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