Water isobutene conversion

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. 

In a typical process, the conversion of isobutene in the reactor stage is 97 per cent. The product is separated from the unreacted methanol and any C4 s by distillation. The essentially pure, liquid, MTBE leaves the base of the distillation column and is sent to storage. The methanol and C4 s leave the top of the column as vapour and pass to a column where the methanol is separated by absorption in water. The C4 s leave the top of the absorption column, saturated with water, and are used as a fuel gas. The methanol is separated from the water solvent by distillation and recycled to the reactor stage. The water, which leaves the base of the column, is... 

Chain-Transfer with anisole. The phenomenon of chain-transfer, especially with aromatic compounds, has been extensively investigated for the polymerisation of styrene, but there is only one such study with isobutene [13]. Isobutene (0.1 mole/l) was polymerised by titanium tetrachloride (3 x 10 3 mole/l) in methylene dichloride with a constant, low, but unknown concentration of water in the presence of anisole (0.02 to 0.15 mole/l) over the temperature range -9° to -90°. The reactions were stopped at 10-20 per cent conversion by the addition of methanol. 

TBA and isobutene have been compared as the etherifying agent at 60 °C. The initial molar ratio of isobutene to glycerol was 4.0 with Amberlyst A35 as the catalyst. The conversion of glycerol is lower when etherified with TBA than when etherified with isobutene. More hydrocarbons are formed with isobutene than with TBA. But, with TBA, mainly monoethers are formed and valuable triethers are formed only in small amounts. In addition, TBA dehydrates to water, which has an inhibition effect on ion-exchange resin catalysts [23],... 

This is an endothennic conversion, which takes place in the gas phase between 150 and 300 C (preferably at about 275 C), at a pressure as low as possible, but suffident to recover the isobutene in the l uid phase by cooling with water, namely about 0.6. 10 Pa absolute. To avoid dehydration side reactions, operations are conducted in the presence of steam, with a typical H2O/MTBE mole ratio at the reactor inlet of 5/1. As in the steam cracking ofhydrocarbons, this procedure serves to reduce the partial pressure of the components and to fedlitate the production of isobutene and methanoL... 

The work of Marek s group also encompasses an interesting study of the polymerisation of isobutene by ethylaluminium dichloride in heptane between —55 and 21 This system showed all the typical traits of direct initiation through selfionisation. Indeed, addition of small amounts of water produced a decrease in the rate of polymerisation with respect to the dry medium (5 x 10 M residual water claimed). The initial rate of monomer consumption was directly proportional to the monomer concentration and the square of the catalyst concentration. The course of the first part of the polymerisations (up to about 40% yield) was internally first order. As the reaction proceeded further, a sensible deceleration was noticed and indeed incomplete yields were often obtained, confirming a general feature of direct initiation. The authors discussed the mode of initiation in terms of selfionisation of the catalyst followed by the attack of the positive ion onto the isobutene molecule, but did not comment about the origin of the anomalous rate decreases and of the limited conversions. 

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-... 

A problem present in the refinery is that, due to its fast transport in water and low biodegradabUity, MTBE addition to gasoline pool has been banned in some countries (from 2003 in Cahfomia). MTBE is formed by add-catalyzed reaction of isobutene with methanol. Other alcohols could be used to form different oxygenated additives, as discussed below, but the alternative is to use isobutene for conversion into another high octane number component such as isooctane, which could substitute in part the need of the alkylation process and related environmental/safety problems. 

The high degree of self-dissociation of water at high densities leads to catalysis of water elimination from alcohols and the formation of double bonds. In the case of tert-butanol [25], complete conversion to isobutene is achieved in 30 s at subcritical temperatures without addition of acids. In other cases, such as the elimination of water from ethanol [26], propanol [27, 28], glycerol [29], glycol [30], fructose [31,... 

A high conversion level of isobutene (99%) can be reached with a double-stage configuration where, in both stages, water-cooled tubular reactors (WCTR), (1,2), are used for the isobutene dimerization to maintain optimal temperature control inside the catalytic bed. 

Water is available in a large excess, which implies that the reaction can be assumed to follow the first-order kinetics. A packed bed is fed with a liquid mixture containing 2 mol/L isobutene. A 50% conversion level of isobutene is required. 

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