Isomerization process operating

Mobil s Low Pressure Isomerization Process (MLPI) was developed in the late 1970s (123,124). Two unique features of this process are that it is Operated at low pressures and no hydrogen is used. In this process, EB is converted to benzene and diethylbenzene via disproportionation. The patent beheved to be the basis for the MLPI process (123) discusses the use of H-ZSM-5 zeoHte with an alumina binder. The reaction conditions described are start-of-mn temperatures of 290—380°C, a pressure of 273 kPa and WHSV of 5—8.5/h. The EB conversion is about 25—40% depending on reaction conditions, with xylene losses of 2.5—4%. The PX approach to equiHbrium is about 99 ndash 101%. The first commercial unit was Hcensed in 1978. A total of four commercial plants have been built. 

The Hysomer process produces an increase of about 12 octane numbers in suitable naphtha feedstocks. The process can be operated in conjunction with the Isosiv process (Union Carbide Corp.) for the separation of normal and isoparaffins, achieving complete isomerization of a C-5—C-6 stream. The combined process is trade named TIP (total isomerization process), and results in increases in octane numbers of about 20, rather than the 12 obtained with a once-through Hysomet treatment. 

SKIP [Skeletal isomerization process] A process for converting linear butenes into isobutene. Developed by Texas Olefins in the 1990s and operated by that company in Houston, TX. 

Xyloflning [Xylol refining] A process for isomerizing a petrochemical feedstock containing ethylbenzene and xylenes. The xylenes are mostly converted to the equilibrium mixture of xylenes the ethylbenzene is dealkylated to benzene and ethylene. This is a catalytic, vapor-phase process, operated at approximately 360°C. The catalyst (Encilite-1) is a ZSM-5-type zeolite in which some of the aluminum has been replaced by iron. The catalyst was developed in India in 1981, jointly by the National Chemical Laboratory and Associated Cement Companies. The process was piloted by Indian Petrochemicals Corporation in 1985 and commercialized by that company at Baroda in 1991. 

Prior to the introduction of ZSM-5-based xylene isomerization processes, most of the commercial units operated with a... 

This replacemem-of-solvent operation should not be carried out with Li- or NaCsCOCjHj and Li- or NaOaCCl (danger of vigorous decomposition or even explosion ) or acetylidcs with the structure MCsCCaCQ R or MOCCH CHCT R (increased risk of isomerization processes). 

The widely known Wilkinson catalyst is proposed to operate through this reaction mechanism. Computational evaluation of the full catalytic cycle showed that the rate-determining step implies the insertion and the subsequent isomerization process (27). Moreover, this catalyst has the particularity that the reaction mechanism depends on the hydrogen source since a monohydridic route has been proposed when 2-propanol is the hydrogen source (28). 

A summary of commercial operations employing the five butane isomerization processes is given in Table II. The data represent typical wartime process operations rather than characteristics of any specific commercial plant. 

The other commercialized pentane isomerization process is that of the Standard Oil Co. (Indiana) (20). This process differs from the Indiana-Texas butane process in that the aluminum chloride is introduced as a slurry directly to the reactor and that about 0.5% by volume of benzene is added continuously in the feed to suppress side reactions. Temperature, catalyst composition, space velocity, and hydrogen chloride concentration are generally similar to those in the corresponding butane process, but the reactor pressure is about 100 pounds lower. The Pan American Refining Co. operated the Indiana pentane isomerization process commercially during the last nine months of the war and produced about 400 barrels of isopentane per calendar day. 

The naphthene isomerization process has been applied also to the conversion of meth-ylcyclopentane to cyclohexane for subsequent dehydrogenation to benzene. Shell s Wilmington, Calif., refinery has been operating commercial equipment on this basis since March 1950 (18). 

The relative contribution of the two mechanisms to the actual isomerization process depends on the metals and the experimental conditions. Comprehensive studies of the isomerization of n-butenes on Group VIII metals demonstrated179-181 that the Horiuti-Polanyi mechanism, the dissociative mechanism with the involvement of Jt-allyl intermediates, and direct intramolecular hydrogen shift may all contribute to double-bond migration. The Horiuti-Polanyi mechanism and a direct 1,3 sigma-tropic shift without deuterium incorporation may be operative in cis-trans isomerization. 

Numerous other technologies, mainly gas-phase hydrogenations, were developed over the years (Sinclair-Engelhard,333,334 Hytoray,335,336 Arco,337 DSM,338 UOP339). The Arco and DSM processes operate at 400°C and under 25-30 atm. Despite the high temperature, isomerization is negligible because of the very short contact time. 

Over 25 plants are operating the total isomerization process (TIP) and this number is growing continuously. Many millions of tons of isomerized paraffins are produced annually by this process. 

Isomerization processes employing any of these traditional catalysts, except HYSOPAR, require full hydrotreating of the feedstock. As a result, very few traditional isomerization units in the world operate today with more than 1 ppmw sulfur. Furthermore, a catalyst whose activity is heavily impacted by sulfur cannot cope with benzene under high sulfur conditions either. The HYSOPAR catalyst was particularly developed to show evidence of high tolerance to feedstock sulfur and to cope with benzene even at high sulfur conditions. This opens up new opportunities for the refiner to feed benzene to isomerization units. 

The requirements for a new isomerization technology were identified to be (i) tolerance to feed impurities, (ii) robustness, (iii) low capital cost and low permanent operating costs. Reviewing the existing isomerization processes, based on chlorinated alumina catalysts, the following features of an improved catalyst were identified that could lead to a process meeting the company s objectives ... 

The typical operating conditions of xylene and EB isomerization processes are shown in Table 9.3. These conditions minimize the above side reactions. Pressure, temperature and H2/HC ratio are key parameters that define the partial pressure of C8 naphthenes intermediates for EB isomerization. Naphthene cracking and disproportionation/transalkylation are responsible for the C8 aromatics net losses that affect the overall pX yield. The C8 recycled stream from the isomerization unit to the separation unit is three times higher than the fresh feed stream (since there cannot be more than 24% of pX in the C8 aromatic cut after isomerization). This means that each percent of loss in the isomerization unit will decrease the pX yield by 3%. For example, when standard mordenite-based catalysts lead to 4% of net losses, the overall pX yield is roughly 88%. 

Tahle 9.3 Typical operating conditions of Xylene and EB isomerization processes... 

Hysomer [Hydroisomerization] A process for converting K-pentane and K-hexane into branched-chain hydrocarbons. Operated in the vapor phase, in the presence of hydrogen, in a fixed bed of a mordenite catalyst loaded with a platinum. Developed by Shell Oil Company and licensed worldwide through UOP. Used in the Total Isomerization process. Now called the UOP Once-Through (OT) Zeolitic Isomerization process. 

We first review in Part 1 the basics of plantwide control. We illustrate its importance by highlighting the unique characteristics that arise when operating and controlling complex integrated processes. The steps of our design procedure are described. In Part 2, we examine how the control of individual unit operations fits within the context of a plantwide perspective. Reactors, heat exchangers, distillation columns, and other unit operations are discussed. Then, the application of the procedure is illustrated in Part 3 with four industrial process examples the Eastman plantwide control process, the butane isomerization process, the HDA process, and the vinyl acetate monomer process. 

Mobil Chemical has devdoped a xylene isomerization process called LTI (Low Temperature Isomerization) whick in the liquid phase, uses qsedal zeolites as catalysts (ZSM5), commercialized by the designation AP (Aromatics Processing These systems are more active than those the REX type, which are generally proposed for vapor phase operation. 

MobU has developed a process similar to its LTI technique called MVPI (Mobil Vapor Phase Isomerization). It operates in vapor phase above 35(PC in the absence of hydrogen. It uses ZSMll zeolite as catalyst and is adapted to transform feedstock with a high ethylbenzene content Ethylbenzene is disproportionated into benzene and para-diethylbenzene. 

The Isocel process developed by the Shell Oil Company was the first isomerization process to reach commercialization and started operation... 

The other vapor-phase butane-isomerization process was developed cooperatively by the Anglo-Iranian Oil Company and the Standard Oil Development Company. It is similar to the Isocel process, but it turns the volatility of aluminum chloride from a liability into an asset. The first plant was put into operation in October, 1942. This process is described (18) by the simplified flow diagram shown in Figure 15. 

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