Ethylene Optimization

Ethylene is a highly symmetric molecule. Here is the input file for an optimization ol its geometry  

The values of 180° for all three dihedral angles specify the molecule in a planar orientation. 

Exploring Chemistry with Electronic Structure Methods 

We ll now look at the output from the ethylene optimization. After some initial output from the setup portion of the optimization job, Gaussian displays a section like the following for each step (the items pointed to by dotted lines do not appear in terse output)  

Old and new values for-structure variables, in atomic units (bohrs and radians] 

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When all four values in the Converged column are YES, then the optimization is completed and has converged, presumably to a local minimum. For the ethylene optimization, convergence happens after 3 steps ... 

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. 

Since the early 1980s olefin plants in the United States were designed to have substantial flexibiHty to consume a wide range of feedstocks. Most of the flexibiHty to use various feedstocks is found in plants with associated refineries, where integrated olefins plants can optimize feedstocks using either gas Hquids or heavier refinery streams. Companies whose primary business is the production of ethylene derivatives, such as thermoplastics, tend to use ethane and propane feedstocks which minimize by-product streams and maximize ethylene production for their derivative plants. 

The white cell adsorption filter layer is typically of a nonwoven fiber design. The biomaterials of the fiber media are surface modified to obtain an optimal avidity and selectivity for the different blood cells. Materials used include polyesters, eg, poly(ethylene terephthalate) and poly(butylene terephthalate), cellulose acetate, methacrylate, polyamides, and polyacrylonitrile. Filter materials are not cell specific and do not provide for specific filtration of lymphocytes out of the blood product rather than all leukocytes. 

Acrylonitrile—Butadiene—Styrene. ABS is an important commercial polymer, with numerous apphcations. In the late 1950s, ABS was produced by emulsion grafting of styrene-acrylonitrile copolymers onto polybutadiene latex particles. This method continues to be the basis for a considerable volume of ABS manufacture. More recently, ABS has also been produced by continuous mass and mass-suspension processes (237). The various products may be mechanically blended for optimizing properties and cost. Brittle SAN, toughened by SAN-grafted ethylene—propylene and acrylate mbbets, is used in outdoor apphcations. Flame retardancy of ABS is improved by chlorinated PE and other flame-retarding additives (237). 

The quantity of catalyst used for a given plant capacity is related to the Hquid hourly space velocity (LHSV), ie, the volume of Hquid hydrocarbon feed per hour per volume of catalyst. To determine the optimal LHSV for a given design, several factors are considered ethylene conversion, styrene selectivity, temperature, pressure, pressure drop, SHR, and catalyst life and cost. In most cases, the LHSV is ia the range of 0.4—0.5 h/L. It corresponds to a large quantity of catalyst, approximately 120 m or 120—160 t depending on the density of the catalyst, for a plant of 300,000 t/yr capacity. 

Polyester and polyether diols are used with MDI in the manufacture of thermoplastic polyurethane elastomers (TPU). The polyester diols are obtained from adipic acid and diols, such as ethylene glycol, 1,4-butanediol, or 1,6-hexanediol. The preferred molecular weights are 1,000 to 2,000, and low acid numbers are essential to ensure optimal hydrolytic stabihty. Also, caprolactone-derived diols and polycarbonate diols are used. Polyether diols are... 

Methanol to Ethylene. Methanol to ethylene economics track the economics of methane to ethylene. Methanol to gasoline has been flilly developed and, during this development, specific catalysts to produce ethylene were discovered. The economics of this process have been discussed, and a catalyst (Ni/SAPO 34) with almost 95% selectivity to ethylene has been claimed (99). Methanol is converted to dimethyl ether, which decomposes to ethylene and water the method of preparation of the catalyst rather than the active ingredient of the catalyst has made the significant improvement in yield (100). By optimizing the catalyst and process conditions, it is claimed that yields of ethylene, propylene, or both are maximized. This is still in the bench-scale stage. 

Process Safety Considerations. Unit optimization studies combined with dynamic simulations of the process may identify operating conditions that are unsafe regarding fire safety, equipment damage potential, and operating sensitivity. Several instances of fires and deflagrations in ethylene oxide production units have been reported in the past (160). These incidents have occurred in both the reaction cycle and ethylene oxide refining areas. Therefore, ethylene oxide units should always be designed to prevent the formation of explosive gas mixtures. 

Recycle reactors at that time were called Backmix Reactors. They were correctly considered the worst choice for the production of a reactive intermediate, yet the best for kinetic studies. The aim of the kinetic study for ethylene oxidation was to maximize the quality of the information, leaving the optimization of production units for a later stage in engineering studies. The recycle reactors could provide the most precise results at well defined conditions even if at somewhat low selectivity to the desired product. 

Our second example takes another member of the vinyl series, and considers the effect of replacing one of the hydrogens in ethylene with a fluorine. The fluoroethylene optimization converges at step 5. By looking at the optimized parameters for each job, we can compare the structures of the two molecules ... 

Compute the isomerization energy between acetaldehyde and ethylene oxide at STP with the QCISD(T)/6-31G(d) model chemistry, and compare the performance of the various model chemistries. Use HF/6-31G(d) to compute the thermal energy corrections. Remember to specify the scaling factor via the Freq=Recxllso option. (Note that we have already optimized the stmcture of acetaldehyde.)... 

Let us finally consider two Z-matrices for optimization to transition structures, the Diels-Alder reaction of butadiene and ethylene, and the [l,5]-hydrogen shift in Z-1,3-pentadiene. To enforce the symmetries of the TSs (Cj in both cases) it is again advantageous to use dummy atoms. 

It is noteworthy that the best results could be obtained only with very pure ionic liquids and by use of an optimized reactor set-up. The contents of halide ions and water in the ionic liquid were found to be crucial parameters, since both impurities poisoned the cationic catalyst. Furthermore, the catalytic results proved to be highly dependent on all modifications influencing mass transfer of ethylene into the ionic catalyst layer. A 150 ml autoclave stirred from the top with a special stirrer... 

Picciotti, M., Optimize Ethylene Plant Refrigeration, Hydrocarbon Processing, V. 58, p. 157, May (1979). 

The preparation and properties of a novel, commercially viable Li-ion battery based on a gel electrolyte has recently been disclosed by Bellcore (USA) [124]. The technology has, to date, been licensed to six companies and full commercial production is imminent. The polymer membrane is a copolymer based on PVdF copolymerized with hexafluoropropylene (HFP). HFP helps to decrease the crystallinity of the PVdF component, enhancing its ability to absorb liquid. Optimizing the liquid absorption ability, mechanical strength, and processability requires optimized amorphous/crystalline-phase distribution. The PVdF-HFP membrane can absorb plasticizer up to 200 percent of its original volume, especially when a pore former (fumed silica) is added. The liquid electrolyte is typically a solution of LiPF6 in 2 1 ethylene carbonate dimethyl car-... 

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