Phase desulfurization

As outlined above, most of the sulfur released from the feed will eventually appear in the gas phase. The primary sulfur problem is synfuels production, therefore, is gas-phase desulfurization. 

Two different approaches are being considered for on-board sulfur removal. The first approach involves liquid-phase desulfurization of the fuel prior to reforming using a nickel-based adsorbent to remove organosulfur compounds (Bonville et al. 

While liquid-phase desulfurization can significantly reduce the sulfur content of the fuel, it is not clear whether the extent of desulfurization achieved by this process will meet the requirements for the fuel processor and fuel cell stack without additional removal of sulfur in the fuel processor. 

The second approach involves gas-phase desulfurization of reformate in the fuel processor using a metal oxide adsorbent, typically ZnO, to remove H2S. Gas-phase desulfurization requires that the reforming catalyst be sulfur tolerant at least to the concentration of sulfur present in the fuel. 

Gas-phase desulfurization using ZnO is the approach that we have pursued in past years. ZnO is an attractive adsorbent for on-board fuel processing because of its favorable sulfidation thermodynamics (<1 ppmv). The sulfidation equilibrium for ZnO (ZnO + H2S o ZnS + H2O) is a function of the temperature and the ratio of the partial pressure of H2O/H2S. To reduce the H2S concentration to <0.1 ppmv requires a temperature of 300-350°C based on the typical range of H2O concentrations present in the reformate (Carter et al. 2001). However, it has been observed that although the equilibrium becomes more favorable as the... 

The main justification for diesel fuel desulfurization is related to particulate emissions which are subject to very strict rules. Part of the sulfur is transformed first into SO3, then into hydrated sulfuric acid on the filter designed to collect the particulates. Figure 5.21 gives an estimate of the variation of the particulate weights as a function of sulfur content of diesel fuel for heavy vehicles. The effect is greater when the test cycle contains more high temperature operating phases which favor the transformation of SO2 to SO3. This is particularly noticeable in the standard cycle used in Europe (ECE R49). 

Naphtha desulfurization is conducted in the vapor phase as described for natural gas. Raw naphtha is preheated and vaporized in a separate furnace. If the sulfur content of the naphtha is very high, after Co—Mo hydrotreating, the naphtha is condensed, H2S is stripped out, and the residual H2S is adsorbed on ZnO. The primary reformer operates at conditions similar to those used with natural gas feed. The nickel catalyst, however, requires a promoter such as potassium in order to avoid carbon deposition at the practical levels of steam-to-carbon ratios of 3.5—5.0. Deposition of carbon from hydrocarbons cracking on the particles of the catalyst reduces the activity of the catalyst for the reforming and results in local uneven heating of the reformer tubes because the firing heat is not removed by the reforming reaction. 

Utihties that reduce emissions below the number of allowances they hold may trade emissions credits on the open market. Owners of plants affected by Phase I regulations can also petition the EPA for a two-year extension for meeting Phase I emissions if they have selected a control option capable of reducing SO2 emissions by 90% or more, such as is capable by flue-gas desulfurization. Owners of these units can receive bonus allowances for 1997—1999 if they have operated at SO2 emissions below 0.52 kg/10 kj (1.2 lb/10 Btu) of fuel heating value input. 

S-Alkylthiiranium salts, e.g. (46), may be desulfurized by fluoride, chloride, bromide or iodide ions (Scheme 62) (78CC630). With chloride and bromide ion considerable dealkylation of (46) occurs. In salts less hindered than (46) nucleophilic attack on a ring carbon atom is common. When (46) is treated with bromide ion, only an 18% yield of alkene is obtained (compared to 100% with iodide ion), but the yield is quantitative if the methanesulfenyl bromide is removed by reaction with cyclohexene. Iodide ion has been used most generally. Sulfuranes may be intermediates, although in only one case was NMR evidence observed. Theoretical calculations favor a sulfurane structure (e.g. 17) in the gas phase, but polar solvents are likely to favor the thiiranium salt structure. 

A packed column is used in the desulfurization operation described in the previous problem. The overall height of transfer unit based on the gas phase is 0.7 m. WTien the flowrate of water is twice the minimum, what height of packing is needed ... 

Glaser and Litt (G4) have proposed, in an extension of the above study, a model for gas-liquid flow through a b d of porous particles. The bed is assumed to consist of two basic structures which influence the fluid flow patterns (1) Void channels external to the packing, with which are associated dead-ended pockets that can hold stagnant pools of liquid and (2) pore channels and pockets, i.e., continuous and dead-ended pockets in the interior of the particles. On this basis, a theoretical model of liquid-phase dispersion in mixed-phase flow is developed. The model uses three bed parameters for the description of axial dispersion (1) Dispersion due to the mixing of streams from various channels of different residence times (2) dispersion from axial diffusion in the void channels and (3) dispersion from diffusion into the pores. The model is not applicable to turbulent flow nor to such low flow rates that molecular diffusion is comparable to Taylor diffusion. The latter region is unlikely to be of practical interest. The model predicts that the reciprocal Peclet number should be directly proportional to nominal liquid velocity, a prediction that has been confirmed by a few determinations of residence-time distribution for a wax desulfurization pilot reactor of 1-in. diameter packed with 10-14 mesh particles. 

A strain P. delafeildii R-8 was reported to desulfurize DBT giving 2-HBP as the end product [92], This strain was isolated from sewage pool of Shanghai oil field. The report described the effect of cell density, oil/water phase ratio, which was very similar to that of IGTS8. These parameters will be discussed in Section 2.2.10. A Michaelis-Menten model was used to describe the kinetics and the Vmax and Km were reported for DBT to be 13mmol/kg dcw/h and 1.3 mM, respectively. 

In addition to desulfurization activity, several other parameters are important in selecting the right biocatalyst for a commercial BDS application. These include solvent tolerance, substrate specificity, complete conversion to a desulfurized product (as opposed to initial consumption/removal of a sulfur substrate), catalyst stability, biosurfactant production, cell growth rate (for biocatalyst production), impact of final desulfurized oil product on separation, biocatalyst separation from oil phase (for recycle), and finally, ability to regenerate the biocatalyst. Very few studies have addressed these issues and their impact on a process in detail [155,160], even though these seem to be very important from a commercialization point of view. While parameters such as activity in solvent or oil phase and substrate specificity have been studied for biocatalysts, these have not been used as screening criteria for identifying better biocatalysts. 

Apart from biocatalyst activity, several other parameters are important in development of a biodesulfurization process. These parameters include oil/water ratio, composition of aqueous phase used for biocatalyst suspension during desulfurization, biocatalyst loading, oil/water separation following completion of desulfurization, potential for biocatalyst recycle, recycle of aqueous phase to reduce fresh water usage and wastewater minimization, as well as secondary oil separation and purification operations. 

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