Paraffins cracking

MIDW [Mobil isomerization dewaxing] A petroleum refining process which improves yield and quality by isomerizing and selectively cracking paraffins in waxy oils. The catalyst is a noble metal, supported on a zeolite. Developed by Mobil Corporation from 1991 to 1996. 

Linear olefins are prepared by dehydrogenation of paraffins, by polymerization of ethylene to a-olefins using a triethyl aluminum catalyst (Ziegler-type catalyst), by cracking paraffin wax, or by dehydrohalogena-tion of alkyl halides. 

Application To produce polymer-grade ethylene and propylene by thermally cracking paraffinic feedstocks (ethane through hydrocracked residue). Two main process technologies are used ... 

The metal catalyst cracks paraffinic chains longer than C25 and reforms chains shorter than Ce. This is especially important to convert the a-olefin chains (1-alkenes) to saturated alkanes. The catalyst ensures that the final fuel has a carbon chain distribution in the range C8-C25 peaking at Cie (cetane) (Figure 15.8). The catalytic tower uses technology borrowed from the petrochemical industry for the hydrogenated of C=C double bonds, e.g. Raney Nickel or so-called Adams catalyst. 

Catalytic dewaxing, developed in the 1970s by Mobil s immensely successful research program into zeolites, specifically ZSM-5 for this application, cracks paraffins and paraffinic groups in wax into light hydrocarbons and is applicable to the entire lube slate. 

The high selectivity indicates that under the reaction conditions employed the types of carbonium ions that will crack are restricted. The product composition indicates that (1) the predominant cracking occurs with tertiary ions which can produce another tertiary ion by -scission (for example, I) and (2) that isomerization equilibrium of the products from cracking (paraffins and lighter cycloparaffins) is not established, presumably because of rapid displacement from the catalyst surface by heavier hydrocarbons. 

Selectively cracking paraffins to light olefins in the reformate which subsequently react with benzene and toluene in the feed (M-forming). 

So-called centre cracking produces a C4-C5 olefin fraction which rapidly isomerises to isobutene and isoamylene. These products were converted to methyl tertiary butyl ether (MTBE) and tertiary amyl methyl ether (TAME) by reaction with methanol to produce octane-enhancing additives for use in reformulated gasoline. Propane and n-butane are also produced. Fresh ZSM-5 also cracks paraffins imtil the acid site density decreases. Eventually, olefin cracking activity dechnes but isomerization activity is retained. Regular addition of fresh ZSM-5 is therefore required to maintain the shape-selective activity. 

The original method for the manufacture of ethyne, the action of water on calcium carbide, is still of very great importance, but newer methods include the pyrolysis of the lower paraffins in the presence of steam, the partial oxidation of natural gas (methane) and the cracking of hydrocarbons in an electric arc. 

The feedstock usually comes from catalytic cracking, sometimes from steam cracking. The reaction products are Cy-Cg isoparaffins. The byproducts are the C3-C4 n-paraffins which do not react. 

Properly speaking, steam cracking is not a refining process. A key petrochemical process, it has the purpose of producing ethylene, propylene, butadiene, butenes and aromatics (BTX) mainly from light fractions of crude oil (LPG, naphthas), but also from heavy fractions hydrotreated or not (paraffinic vacuum distillates, residue from hydrocracking HOC). 

In a single stage, without liquid recycle, the conversion can be optimized between 60 and 90%. The very paraffinic residue is used to make lubricant oil bases of high viscosity index in the range of 150 N to 350 N the residue can also be used as feedstock to steam cracking plants providing ethylene and propylene yields equal to those from paraffinic naphthas, or as additional feedstock to catalytic cracking units. 

Mobil s High Temperature Isomerization (MHTI) process, which was introduced in 1981, uses Pt on an acidic ZSM-5 zeoHte catalyst to isomerize the xylenes and hydrodealkylate EB to benzene and ethane (126). This process is particularly suited for unextracted feeds containing Cg aHphatics, because this catalyst is capable of cracking them to light paraffins. Reaction occurs in the vapor phase to produce a PX concentration slightly higher than equiHbrium, ie, 102—104% of equiHbrium. EB conversion is about 40—65%, with xylene losses of about 2%. Reaction conditions ate temperature of 427—460°C, pressure of 1480—1825 kPa, WHSV of 10—12, and a H2/hydtocatbon molar ratio of 1.5—2 1. Compared to the MVPI process, the MHTI process has lower xylene losses and lower formation of heavy aromatics. 

Aliphatic C-5—C-6. Aliphatic feedstreams are typically composed of C-5 and C-6 paraffins, olefins, and diolefins, the main reactive components being piperylenes cis-[1574-41 -0] and /n j -l,3-pentadiene [2004-70-8f). Other main compounds iaclude substituted C-5 and C-6 olefins such as cyclopentene [142-29-OJ, 2-methyl-2-butene [513-35-9] and 2-methyl-2-pentene [625-27-4J. Isoprene and cyclopentadiene maybe present ia small to moderate quaatities (2—10%). Most steam cracking operatioas are desigaed to remove and purify isoprene from the C-5—C-6 fraction for applications ia mbbers and thermoplastic elastomers. Cyclopentadiene is typically dimerized to dicyclopentadiene (DCPD) and removed from C-5 olefin—diolefin feedstreams duriag fractionation (19). 

Low temperature filtration (qv) is a common final refining step to remove paraffin wax in order to lower the pour point of the oil (14). As an alternative to traditional filtration aided by a propane or methyl ethyl ketone solvent, catalytic hydrodewaxing cracks the wax molecules which are then removed as lower boiling products. Finished lubricating oils are then made by blending these refined stocks to the desired viscosity, followed by introducing additives needed to provide the required performance. Table 3 Usts properties of typical commercial petroleum oils. Methods for measuring these properties are available from the ASTM (10). 

Catalysis. As of mid-1995, zeoHte-based catalysts are employed in catalytic cracking, hydrocracking, isomerization of paraffins and substituted aromatics, disproportionation and alkylation of aromatics, dewaxing of distillate fuels and lube basestocks, and in a process for converting methanol to hydrocarbons (54). 

Wax Cracking. One or more wax-cracked a-olefin plants were operated from 1962 to 1985 Chevron had two such plants at Richmond, California, and Shell had three in Europe. The wax-cracked olefins were of limited commercial value because they contained internal olefins, branched olefins, diolefins, aromatics, and paraffins. These were satisfactory for feed to alkyl benzene plants and for certain markets, but unsatisfactory for polyethylene comonomers and several other markets. Typical distributions were C 33% C q, 7% 25% and 35%. Since both odd and... 

Catalytic dewaxiag (32) is a hydrocrackiag process operated at elevated temperatures (280—400°C) and pressures, 2,070—10,350 kPa (300—1500 psi). However, the conditions for a specific dewaxiag operatioa depead oa the aature of the feedstock and the product pour poiat required. The catalyst employed for the process is a mordenite-type catalyst that has the correct pore stmcture to be selective for normal paraffin cracking. Platinum on the catalyst serves to hydrogenate the reactive iatermediates so that further paraffin degradation is limited to the initial thermal reactions. 

Another catalytic dewaxiag process also iavolves selective cracking of aormal paraffias and those paraffins that may have minor branching ia the chaia. la the process (Fig. 8), the catalyst can be reactivated to fresh activity by relatively mild nonoxidative treatment. The time allowed between reactivations is a function of the feedstock after numerous reactivations it is possible that there will be coke buildup on the catalyst. 

Catalytic Reforming. Worldwide, approximately 30% of commercial benzene is produced by catalytic reforming, a process ia which aromatic molecules are produced from the dehydrogenation of cycloparaffins, dehydroisomerization of alkyl cyclopentanes, and the cycHzation and subsequent dehydrogenation of paraffins (36). The feed to the catalytic reformer may be a straight-mn, hydrocracked, or thermally cracked naphtha fraction ia the... 

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