Desulfurization rate

IMP-S02 [49], and ECRD-1 [50], Many of these strains are similar in the mechanism of DBT desulfurization, but vary in terms of the desulfurization rate as well as physiological characteristics. 

A comparison of the specific desulfurization rate of various Rhodococcus species in whole cells is given in Table 2. The rate for the IGTS8 strain, which has been studied the most, has ranged between 6-72 mmol/kg dcw/h, with average rate reported being around 30 mmol/kg dcw/h. The variation may be due to the experimental conditions employed... 

Table 2. Comparison of specific desulfurization rates of various Rhodococcus strains...

Strain Desulfurization rate, mmol/Kg dcw/h or as noted Notes/assay conditions Ref. 

Further research in improving the BDS activity of the biocatalysts was targeted towards the search of co-catalysts and co-factors to enhance overall desulfurization rates as well as promoters to enhance enzyme expression. This research resulted in identification of NADH and FMNH2 as co-factors essential for electron transfer and related oxidoreductase enzymes as co-catalysts as described in detail below. Additionally, other bacterial strains were also investigated as hosts and are reported below. 

Table 11. Comparison of desulfurization rates of various alkyl DBT substrates by various native and recombinant Rhodococcus sp.

In this subsection, specific operating parameters will be discussed. These includes nutrient media for use during desulfurization process, toxicity of solvent to various biocat-alytic strains, effect of oil/water ratio on biocatalyst as well as desulfurization rates and subsequent separations, biocatalyst density during desulfurization, and finally, the alternative to operate the desulfurization process in batch vs. continuous mode. 

The effect of oil/water ratio has been studied extensively for various catalysts. Patel et al. [258] reported effect of oil/water ratio on rate of desulfurization by IGTS8. They used freeze-dried cells reconstituted with water to do the studies. They found that a minimum of 1.25 mL of water per gram of freeze-dried cells is necessary to enable biodesulfurization. At a W/O ratio of 1 9, about 82% of the maximum desulfurization activity was achieved. The rate of desulfurization was reported to be similar between the W/O ratio of 1 1 and 4 1, but decreased upon increasing oil content further. The effect of the ratio was also studied by Shan et al. using diesel oil as the oil phase and P. delafieldii R-8 as the biocatalyst [259], The water content was varied to obtain a W/O ratio between 0 1 and 20 1, using a fixed amount of biocatalyst and oil. The authors found that the desulfurization rate increased up to a W/O ratio of 2 1, after which it remained constant. 

The effect of the cell density was studied in biodesulfurization of diesel oil by P. delafieldii R-8 [259], An optimum was reported to exist for this biocatalyst as well. Above 25g/L cell density, the specific desulfurization rate decreased. In this case a statistical analysis was not performed to identify the point of mass transfer limitation. 

Use of a N. globerula R-9 strain was demonstrated for desulfurization of straight run diesel oils. Sulfur reduction from 1807 to 741 mg/dm3 was reported at a desulfurization rate of 5.1 mmol/Kgdcw/h. The desulfurization of model oils containing DBT and 4,6 dimethyl DBT was studied and Michaelis-Menten kinetic parameters were reported. 

A 20% excess of hexamethylphosphorus triamide is utilized to increase the reaction rate. When a 10% excess of the phosphine is used, the reaction is not quite complete (as monitored by gas chromatography) even after 3 hours. If the reaction is not allowed to go to completion, the chromatographed product will contain benzyl disulfide. Note that the desulfurization rate for hexaethyl-phosphorus triamide is comparable to that for hexamethylphosphorus triamide. However, the chromatographic separation following the reaction is much more efficient when hexamethylphosphorus triamide is used. 

Most researchers have found pseudo-first-order behavior for the various steps, and so it is possible to match theoretical curves with data to obtain the best rate constant values. Unfortunately, in most instances, too few data points were obtained to generate a unique theoretical fit. It is absolutely imperative that data be obtained for at least four conversion levels that are well spaced in the conversion matrix and extend to over 95% conversion. The partially hydrogenated dibenzothiophene intermediates are most often never detected as their desulfurization rates are extremely high (fcD, and kn2). The cyclohexylbenzenes and bicyclohexyls can arise from two different routes, and the concentrations of their precursors (biphenyl and cyclohexyl-biphenyl, respectively) pass through maximum values that can easily be calculated from the relative values of the formation and conversion rate constants. However, unique values for these relative rates can only be predicted if data are available well prior to and well beyond the times of maximum concentrations for these intermediates, because minor experimental errors can confuse curve-fitting optimization. 

As mentioned earlier, the desulfurization rates of tetra- and hexahydrodi-benzothiophenes are high relative to rates of other reactions in the overall HDS reaction scheme. Because of this, these intermediates are often not observed experimentally. Thus, the origins of cyclohexylbenzene and bicyclohexane are confused. When attempting to deconvolute all of the rate... 

As discussed in a later section, H2S is an inhibitor for the catalytic site responsible for direct sulfur extraction. Thus, if the H2S partial pressure could be lowered in the reactor, the desulfurization rate could be increased. The simplest means to achieve this goal is through increased hydrogen recycle rates or increasing the hydrogen/feed ratio. Such changes are expensive and can in some instances lower the overall thoughput of the feed. 

Table 9-20 Variation of Desulfurization Rate with Sulfur Content for the Unicracking/HDS Process... 

Crude source Crude (vol%) Gravity (°API) Sulfur (wt%) Ni + V (ppm) carbon (wt%) desulfurization rate... 

Figure 9-31 Variation of desulfurization rate with sulfur content of the feedstock (unicracking/HDS) process.

Another slurry pipeline desulfurization experiment was conducted using Indiana 3 (Ayrshire) coal as a 25 wt% slurry in deionized water. The other process variables were carefully controlled flow rates 6-6.5 ft/sec, temperature 70-90°F, and pH 2.5 -2 8.The experiment was continued for 14 days, and the slurry samples for pyritic sulfur determination were taken daily. The desulfurization rates with Indiana 3 coal in the pipeline experiment are shown in Table 4 and are in good agreement with the laboratory data and the results with Illinois 6 coal. As observed in the laboratory experiments, the rate of desulfurization of bituminous coals is directly proportional to the pyritic sulfur content and inversely to the particle size of the coal sample. 

To control the exotbeimicity of the reaction, a number of technological arrangements are indispensable, sudi as intermediate quenches in the reactor. To maintain a sat actory desulfurization rate, this must therefore be compensated by a lower space velocity or better activity of the catalyst system, especially since the ri of polymerization attributable to the residual diolefms and olefins, which are hydrogenated hrst, require operation at the top of the reactor at the lowest possible temperature and with a high panial pressure of hydrogen. 

Figure 2 shows that the desulfurization rate is a function of the Al/P ratio for CoMo/AAP catalysts. Under the reaction conditions, all catalysts showed a high initial activity followed by a deactivation. The decrease in activity during the initial period of time is very probably connected with coke formation (ref. 6). Although the poisoning effect is not entirely excluded for deactivation, its contribution is far smaller than coke in the initial period of time. 

Desulfurization rate, second order, 1000°F inhibited -rs = flks(l-Ca/CaJ ... 

Table I. Effect of the Catalyst-to-Dibenzothiophene Ratio on the Desulfurization Rate at 78.8 °C for 60 min...

Eisch et al. (24) performed a mechanistic study of the desulfurization of dibenzothiophene by a nickel(0)-bipyridyl complex and reported that a radical anion of the thiophene nucleus was formed and underwent C-S bond cleavage into S and an aromatic radical. In addition, they suggested that the oxidative reaction of the nickel(0)-bipyridyl complex toward dibenzothiophene had the characteristics of stepwise electron transfer rather than nucleophilic attack. However, no correlations occurred between the desulfurization rate and the reaction indexes of Fr(E), Fr(N), and Fr(R), as shown in Table II. The results suggested no evidence for either electron transfer or nucleophilic attack in this study. Moreover, the radical reaction was not... 

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