Cracking of MTBE

There are currentiy three important processes for the production of isobutylene (/) the extraction process using an acid to separate isobutylene (2) the dehydration of tert-huty alcohol, formed in the Arco s Oxirane process and (3) the cracking of MTBE. The expected demand for MTBE wHl preclude the third route for isobutylene production. Since MTBE is likely to replace tert-huty alcohol as a gasoline additive, the second route could become an important source for isobutylene. Nevertheless, its avaHabHity wHl be limited by the demand for propylene oxide, since it is only a coproduct. An alternative process is emerging that consists of catalyticaHy hydroisomerizing 1-butene to 2-butenes (82). In this process, trace quantities of butadienes are also hydrogenated to yield feedstocks rich in isobutylene which can then be easHy separated from 2-butenes by simple distHlation. 

Derivation Fractionation of refinery gases, catalytic cracking of MTBE. 

Olefin isomerization reactions range from some of the most facile using acid catalysts to moderately difficult and, as components of more complex reaction schemes such as catalytic cracking, may be among the most common reactions in hydrocarbon processing. As stand-alone reactions, they are primarily used to shift the equilibrium between terminal and internal olefins or the degree of branching of the olefin. While olefin isomerization was considered for the production of MTBE, today stand-alone olefin isomerization processes are only considered for a few special situations within a petrochemical complex. 

In practice, short-chain alkanes and alkenes are normally used as feedstock for shape-selective catalytic formation of isooctanes at relatively low temperatures. Until the 1980s, lead alkyls (Section 18.1) were added to most automotive fuels to help suppress engine knock, but they have been phased out in North America because of the chronic toxicity of lead and lead compounds. The most commonly used nonlead antiknock additive is now methyl tert-butyl ether [MTBE CH30C(CH3)3], which is made by the reaction of methanol with 2-methylpropene, (CHs C—CH2 (see Section 7.4). The latter is obtained by catalytic cracking of petroleum fractions to give 1-butene, which is then shape-selectively isomerized on zeolitic catalysts. 

The cracking of isobutane to isobutylene is of special interest because of the high demand for isobutylene as a feed for the production of oxygenates, mainly MTBE, as octane-enhancing gasoline additives. Isobutane separated from the steam cracking C4 cut or produced by butane isomerization is cracked in the presence of steam to yield isobutylene and propylene.148,149... 

Natural gas liquids represent a significant source of feedstocks for the production of important chemical building blocks that form the basis for many commercial and industrial products. Ethylene (qv) is produced by steam-cracking the ethane and propane fractions obtained from natural gas, and the butane fraction can be catalytically dehydrogenated to yield 1,3-butadiene, a compound used in the preparation of many polymers (see Butadiene). The -butane fraction can also be used as a feedstock in the manufacture of MTBE. 

The introduction of catalytic converters has had a tremendous impact on the composition of gasoline. The catalysts used became poisoned by small amounts of impurities in particular the lead compounds present in high octane gasoline were detrimental. Processes which produce high octane number compounds were therefore stimulated. First, cracking and reforming increased in importance. More recently, the aromatics content is also expected to have to decrease and alternative processes are in use or under way, e.g. the production of MTBE (methyl tertiary-butyl ether). 

Isobutene is present in refinery streams. Especially C4 fractions from catalytic cracking are used. Such streams consist mainly of n-butenes, isobutene and butadiene, and generally the butadiene is first removed by extraction. For the purpose of MTBE manufacture the amount of C4 (and C3) olefins in catalytic cracking can be enhanced by adding a few percent of the shape-selective, medium-pore zeolite ZSM-5 to the FCC catalyst (see Fig. 2.23), which is based on zeolite Y (large pore). Two routes lead from n-butane to isobutene (see Fig. 2.24) the isomerization/dehydrogenation pathway (upper route) is industrially practised. Finally, isobutene is also industrially obtained by dehydration of f-butyl alcohol, formed in the Halcon process (isobutane/propene to f-butyl alcohol/ propene oxide). The latter process has been mentioned as an alternative for the SMPO process (see Section 2.7). 

The main use of MTBE is as an octane booster in gasoline formulations. Table 2.1 (above) compares octane number and boiling points of some tertiary ethers and hydrocarbons. The volatility is another important property of gasoline components. In fact, the lower volatility of ETBE is an advantage with respect to MTBE. Another (smaller scale) application of MTBE is the synthesis of high purity isobutene by cracking MTBE over amorphous silica-alumina. This isobutene serves as a monomer for polyisobutene. 

The main by products of MTBE cracking are dimethyl ether obtained by methanol dehydration, the dimer and ttimer of isobutene, and t-butyl alcohol resulting from the polymerization and hydradoo of the olefin. 

The continuous increase in world consumption of MTBE has created a strong incentive to increase the production of isobutylene. Isobutylene can be produced by catalytic dehydrogenation of isobutane. However, the largest production of C4 olefins comes from the thermal cracking processes for the manufacture of ethylene which generate as by-products C4 mixtures containing C4 olefins and C4 alkanes plus butadiene. Isobutylene is also a product of fluid bed catalytic cracking units. 

These zirconium phosphate materials are being developed as replacements for ion exchange resin catalysts. The arylsulfonic acid MELS have been evaluated for butene isomerization, methanol dehydration, MTBE synthesis as well as cracking, and for the alkylation of aromatics. In the synthesis of MTBE this catalyst appears to out-perform the ion exchange resins, Amberlyst 15. 

This is an endothermic conversion, which takes place in the gas phase between 150 and 300 0 (preferably at about 275 0), at a pressure as low as possible, but sufficient to recover the isobutene in the liquid phase by cooling with water, namely about 0.6I06 Pa absolute. To avoid dehydration side reactions, operations are conducted in the presence of steam, with a typical H20/MTBE mole ratio at the reactor inlet of 5/1. As in the steam cracking of hydrocarbons, this procedure serves to reduce the partial pressure of the components and to facilitate the production of isobutene and methanol. 

Cj olefmic components find fewer applications than the C4 compounds. The main applications concern isoamylenes with a tertiary carbon atom, ix. essentially 2-methyl butenes, which produce isoprene by dehydrogenation and Ter Amyl Methyl Ether (TAME) by etherification. The fust conversion is discussed in detail in Section 6. As for the second its value, like that of MTBE, is associated with the antiknock properties of TAME which make it an excellent octane promoter for gasolines. By cracking, the ether can even reproduce isoamylenes. This operation offers one means of separating 2-metbyl butenes and a method that is likely to be more economic than direct extraction to obtain isoprene. 

Methyl /-Butyl Ether. MTBE is produced by reaction of isobutene and methanol on acid ion-exchange resins. The supply of isobutene, obtained from hydrocarbon cracking units or by dehydration of tert-huty alcohol, is limited relative to that of methanol. The cost to produce MTBE from by-product isobutene has been estimated to be between 0.13 to 0.16/L ( 0.50—0.60/gal) (90). Direct production of isobutene by dehydrogenation of isobutane or isomerization of mixed butenes are expensive processes that have seen less commercial use in the United States. 

Production of maleic anhydride by oxidation of / -butane represents one of butane s largest markets. Butane and LPG are also used as feedstocks for ethylene production by thermal cracking. A relatively new use for butane of growing importance is isomerization to isobutane, followed by dehydrogenation to isobutylene for use in MTBE synthesis. Smaller chemical uses include production of acetic acid and by-products. Methyl ethyl ketone (MEK) is the principal by-product, though small amounts of formic, propionic, and butyric acid are also produced. / -Butane is also used as a solvent in Hquid—Hquid extraction of heavy oils in a deasphalting process. 

The alkylation unit in a petroleum refinery is situated downstream of the fluid catalytic cracking (FCC) units. The C4 cut from the FCC unit contains linear butenes, isobutylene, n-butane, and isobutane. In some refineries, isobutylene is converted with methanol into MTBE. A typical modern refinery flow scheme showing the position of the alkylation together with an acid regeneration unit is displayed in Fig. 1. 

A more recent raw material for plasticizer alcohols is crack-C4 as a byproduct of steamcrackers in ethene/propene production. After extraction of butadiene for use and etherification of isobutene with methanol to methyl-tertiary-butylether MTBE as an octane enhancer, a stream is left containing 1-butene, 2-butene, and butanes, so-called raffinate II. Oligomerization of the butenes yields C8 olefin mixtures ( dibutene ) as the main product and the corresponding C12 olefins as the main byproduct (tributene). They are the... 

Isobutylene has had a tremendous increased production in the last few years because of the dynamic growth of the gasoline additive MTBE. About two thirds of it is made from isobutane by dehydrogenation in thermal cracking. 

The catalytic alkylation of isobutane with C3—C5 alkenes was commercialized in the US during WW II. Blending the alkylate product with catalytically cracked gasoline provided high-octane aviation fuel. The introduction of aromatic and oxygenated fuel additives, such as methyl t-butyl ether (MTBE), pushed alkylation to the sidelines. However, in the 1990s, when the environmental effects of such additives were realized, alkylation regained its importance [191]. 

---