Tailoring Reaction and Processing Conditions

Some of these factors are elaborated further below. It should be noted that conventional approaches for fuel desulfurization in response to the 1993 diesel fuel sulfhr regulation (500 ppmw sulfur) in the U S. were to increase process severity of HDS, increase catalysts to fuel ratio, increase residence time, and enhance hydrogenation, or to use additional low-sulfur blending stocks either from separate process streams or purchased. It is becoming more difficult to meet the ultra-low-sulfur fuel specifications by fuel hydrodesulfurization using the conventional approaches. 

Liquid-hourly space velocity (LHSV) and catalytic bed volume are interrelated parameters that control both the level of sulfur reduction and the process throughput. Increase in catalyst bed volume can enhance desulfurization. UOP projects that doubling reactor volume would reduce sulfur from 120 to 30 ppmw . Haldor-Topsoe reports that doubling the catalyst volume results in a 20 °C decrease in average temperature if all other operating conditions are unchanged, and there is a double effect of the increased catalyst volume . The deactivation rate decreases because the 

Increasing the recycle gas/oil ratio (increase in the amount of recycle gas sent to the inlet of the reactor) could increase the degree of desulfurization, but the effect is relatively small, so a relatively large increase is needed to achieve the same effect as scrubbing the recycle gas . Haldor-Topsoe indicates that a 50% increase in the ratio of total gas/liquid ratio only decreases the necessary reactor temperature by 6 to 8°C or the temperature can be maintained and the final sulfur level reduced by 35-45%.  

The improvement in vapor-liquid contact can enhance the performance of distillate hydrotreaters. As an example, in testing of an improved vapor-liquid distributor in commercial use, Haldor-Topsoe and Phillips Petroleum found that the new Topsoe Dense Pattern Flexible Distribution Tray (installed in 1996 to replace a chimney type distributor installed in 1995 in a refiney) allowed a 30% higher sulfur feed to be processed at 25°C lower temperatures, while reducing the sulfur content of the product from 500 to 350 ppmw . Albemarle estimates that an improved vapor-liquid distributor can reduce the temperature necessary to meet a 50 ppmw sulfur level by 10 °C, which in turn would increase catalyst life and allow an increase in cycle length from 10 to 18 months. Based on the above data from Haldor-Topsoe, if temperature were maintained, the final sulfur level could be reduced by 50%. Maintaining temperature should have allowed an additional reduction in sulfur of more than two-thirds. Thus, ensuring adequate vapor-liquid contact can have a major impact on final sulfur levels. 

The above-mentioned individual improvements described cannot be simply combined, either additively or multiplicatively. As mentioned earlier, each existing distillate hydrotreater is unique in its combination of design, catalyst, feedstock, and operating conditions. While the improvements described above are probably indicative of improvements which can be made in many cases, it is not likely that all of the improvements mentioned are applicable to any one unit the degree of improvement at one refinery could either be greater than, or less than the benefits that are indicated for another refinery. 

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Approaches to ultradeep HDS include (a) improving catalytic activity by new catalyst formulation for HDS of 4,6-DMDBT, (b) tailoring reaction and process conditions, and (c) designing new reactor configurations. Design approaches for ultradeep HDS focus on how to remove 4,6-DMDBT in gas oil more effectively. One or more approaches may be employed by a refinery to meet the challenges of producing ultraclean fuels at affordable cost. 

The synthetic sequence to methylene-bridged poly(phenylene)s 71 represents the first successful employment of the stepwise process to ladder-type macromolecules involving backbone formation and subsequent polymer-analogous cyclization. As shown, however, such a procedure needs carefully tailored monomers and reaction conditions in order to obtain structurally defined materials. The following examples demonstrate that the synthesis of structurally defined double-stranded poly(phenylene)s 71 (LPPP) via a non-concerted process is not just a single achievement, but a versatile new synthetic route to ladder polymers. By replacing the dialkyl-phenylenediboronic acid monomer 68 by an iV-protected diamino-phenylenediboronic acid 83, the open-chain intermediates 84 formed after the initial aryl-aryl cross-coupling can te cyclized to an almost planar ladder-type polymer of structure 85, as shown recently by Tour and coworkers [107]. 

Table 2 shows to what extent homogeneous catalysts are tailor-made and how variable and adaptable they are to the problem concerned by suitable reaction and unit processes, taking as examples the modem hydroformylation processes and catalysts. It clearly illustrates that a variety of different solutions in terms of reaction conditions and product separation technologies are available to meet any... 

The characteristics of limited operating regions, substrate or product inhibition, and reactions in aqueous solutions have often been considered as the most serious drawbacks of biocatalysts. Many of these drawbacks, however, turn out to be misconceptions and prejudices.For example, many commercially used enzymes show excellent stability with half-lives of months or even years under process conditions. In addition, there is an enzyme-catalyzed reaction equivalent to almost every type of known organic reaction. Many enzymes can accept non-natural substrates and convert them into desired products. More importantly, almost all of the biocatalyst characteristics can be tailored with protein engineering and metabolic engineering methods (refer to the section Biocatalyst Engineering and see also the entry Protein Design ) to meet the desired process conditions. 

The porosity of the film can be tailored during initial stages and other processing conditions. The initial factors include pH (acid- or base-catalyzed), temperature of reaction, reagent concentrations, and H20/Si molar ratio. The processing conditions that affect the porosity are aging, temperature, and time... 

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