Liquid 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. 

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. 

In order to ascertain whether sufficient nickel to complete a given reaction has been used, the liquid phase may be tested for starting material before an attempt is made to isolate a product. In general the sodium fusion test for sulfur is satisfactory for this purpose but in certain individual cases a specific test may be more convenient. Thus, in the desulfurization of thioacetals (mercaptals), unreacted material... 

At least one developer is developing a liquid-phase fuel desulfurizer cartridge that will be used to remove sulfur prior to fuel vaporization. Other developers remove the sulfur immediately after vaporization and prior to the reforming. Hydrogen needs to be recirculated to the removal device to convert the sulfur species to H2S so that it can be entrapped on zinc oxide, a complication. 

Apart from liquid phase adsorption on a solid adsorbent such as bauxite, the early processes for sweetening and desulfurization were of a chemical nature. Some are in operation today in substantially their original forms, some have been greatly improved, and new processes performing similar functions have been developed. It is beyond the scope of this paper to cover them all, even in outline, therefore a comparative selection has been made to illustrate the advances achieved. The division, which is on a rather arbitrary basis, is given in Table V. 

Oxidation and extraction, that is, liquid-phase oxidation with organic peroxides followed by separation of the oxidized sulfur (oxidative desulfurization, ODS) [51-58]. 

In the process (Figure 9-37), the residue feed is slurried with a small amount of finely powdered additive and mixed with hydrogen and recycle gas prior to preheating. The feed mixture is routed to the liquid phase reactors. The reactors are operated in an up-flow mode and arranged in series. In a once through operation conversion rates of >95% are achieved. Typically the reaction takes place at temperatures between 440 and 480°C and pressures between 150 and 250 bar. Substantial conversion of asphaltenes, desulfurization and denitrogenation takes place at high levels of residue conversion. Temperature is controlled by a recycle gas quench system. 

Equilibrium calculations are useful in the design or operation of a flue gas desulfurization (FGD) facility and provide the necessary foundation for complex process simulation (e.g., absorber modeling) (3). Since S02 absorption into FGD slurries is a mass transfer process which is primarily limited by liquid phase resistance for most commercial applications, the solution composition, in terms of alkaline species, is very critical to the performance of the system. Accurate prediction of solution composition via equilibrium models is essential to establishing driving forces for mass transfer, and ultimately in predicting system performance. 

OATS [Olefinic Alkylation of Thiophenic Sulfur] A gasoline desulfurization process. Thiophenes and mercaptans are catalytically reacted with olefins to produce higher-boiling compounds that can more easily be removed by distillation prior to hydrodesulfurization. This minimizes hydrogen usage. The process uses a solid acid catalyst in a liquid-phase, fixed bed reactor. Developed by BPAmoco in 2000 and tested in Bavaria and Texas. First used commercially at the Bayernoil refinery, Neustadt, in 2001. The process won a European Environment Award in 2002. 

First results on n-complexation sorbents for desulfurization with Ag-Y and Cu(I)-Y zeolites have been reported recently [3,4]. In this work, we included the known commercial sorbents such as Na-Y, Na-ZSMS, H-USY, activated carbon and activated alumina (Alcoa Selexsorb) and made a direct comparison with Cu(l)-Y and Ag-Y which were the sorbents with n-complexation capability. Thiophene and benzene vapors were used as the model system for desulfurization. Although most of these studies can be applied directly to liquid phase problems, Cu-Y (auto-reduced) and Ag-Y zeolites were also used to separate liquid mixtures of thiophene/benzene, thiophene/n-octane, and thiophene/benzene/n-octane at room temperature and atmospheric pressure using fixed-bed adsorption/breakthrough techniques. These mixtures were chosen to understand the adsorption behavior of sulfur compounds present in hydrocarbon liquid mixtures and to study the performance of the adsorbents in the desulfurization of transportation fuels. Moreover, a technique for regeneration of the adsorbents was developed in this study [4]. 

There are many methods for the desulfurization of nature gas, which can be classified into dry desulfurization, wet desulfurization, and catalytic adsorption. In the dry desulfurization, some solid sorbents, such as iron oxide, zinc oxide, activated carbon (AC), zeolites, and molecular sieves, are used. In wet desulfurization method, liquid-phase chemical/physical solvent absorption systems are usually used for scrubbing H2S amine-based processes are subject to equipment corrosion, foaming, amine-solution degradation, and evaporation, and require extensive wastewater treatment. As a result, this sulfur removal technology is complex and capital intensive,44 although the processes are still employed widely in the industry. The desulfurization of coal gasification gas will be reviewed in detail in Section 5.5. In the catalytic-adsorption method, the sulfur compounds are transformed into H2S by catalytic HDS or into elemental sulfur or SOx by selective catalytic oxidation (SCO), and then, the reformed H2S and SOx are removed by the subsequent adsorption. 

Numerous published results indicate activated carbons as efficient adsorbents for desulfurization either from gas or liquid phase. Complex processes take place on their surface leading to adsorption of sulfur containing compounds, their oxidation and deposition of oxidation products in the pore system. For all of these processes, it is the surface features of activated carbon that govern the removal processes. In the majority of cases it is impossible to separate the role of porosity, pore sizes and pore vohune form tlie role of surface chemistry. Since their coexistence is a must on the surface of activated carbons, the way in which they affect the feasibility of desulfurization is a synergy and its is this unique synei which opens the way for the application of carbon surfaces in separation technology and thus environmental remediation. 

A somewhat different type of distillation with reaction is catalytic distillation fPoherty et al.. 2008 Parkinson. 200S1. In this process bales of catalyst are stacked in the column. The bales serve both as the catalyst and as the column packing (see Chapter 101. This process was used commercially for production of methyl tert-butyl ether (MTBE) from the liquid-phase reaction of isobutylene and methanol. The heat generated by the exothermic reaction is used to supply much of the heat required for the distillation. Since MTBE use as a gasoline additive has been oudawed because of pollution problems from leaky storage tanks, these units are shut down. Other applications of catalytic distillation include desulfurization of gasoline, separation of 2-butene from a mixed C4 stream, esterification of fatty acids and etherification. 

The hydrogenation of naphthalene to tetralin is analogous to the hydrogenation of benzene to produce cyclohexane. The reaction can be carried out in the liquid phase with nickel catalysts at 200 °C and 10 to 15 bar hydrogen pressure. The jFeedstock is desulfurized naphthalene, which can be obtained by treatment of crude naphthalene with sodium. 

The first step of electrochemical desulfurization is the adsorption of SO2 into the liquid phase. To increase the solubility of the gases in the liquid phase, a reaction, which converts the primarily dissolved species to a more soluble one, must be used. This can be achieved by either the direct conversion at the electrode or the indirect conversion via chemical reactions with a mediator. 

It is therefore obvious that a desulfurization unit upstream of the autothermal reformer is essential if sulfur-containing fuels are used. As mentioned previously, the upper value for the sulfur mass fraction of the fuel entering the fuel-cell system is 10 ppm [1-3]. In the following sections, different approaches will be presented for the desulfurization of middle distillates in the gas and liquid phases. 

Trickle-bed reactors have the following problems (i) In general they are complicated with respect to the mass transfer (gas-liquid and liquid-solid). In addition, proper hydrodynamic conditions (wetting of catalyst, distribution of gas and liquid phase) are hard to realize. Scale-up is therefore difficult and pilot plants are often still needed, (ii) The amount of H2 fed to the reactor - above all in the case of a second stage deep desulfurization of an already hydrotreated fuel - is much higher than the amount chemically needed. Therefore, a costly recycle compressor for the (unconsumed) hydrogen has to be installed. 

For deep desulfurization, trickle-bed reactor technology may also be improved by using a two-phase reactor. Thereby, the oil is externally pre-saturated with H2 and only the liquid phase is passed over the catalytic fixed bed. To discuss the... 

Gdmez-Bernal, H., Cedeno-Caero, L. and Gutierrez-Alejandre, A. (2009). Liquid phase oxidation of dibenzothiophene with alumina-supported vanadium oxide catalysts An alternative to deep desulfurization of diesel, Catal. Today., 142, pp. Til-Til). 

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