Kinetics, hydrodesulfurization

Figure 5. Hydrodesulfurization kinetics for an Arabian light atmospheric residuum...

In order to determine the rate equation for hydrodesulfurization, a semi-logarithmic plot of the total sulfur content with time was made (Figure 2). The plot indicated two independent first-order reactions with greatly different rate constants. This is in agreement with the findings of Gates, et al. (7) and Pitts (3). A procedure similar to that of Pitts (3 ) was used to describe the hydrodesulfurization kinetics. The rate expression is given below ... 

Fundamental studies of hydrodesulfurization kinetics often use thiophene (C4H4S) to represent the type of sulfur bonding present in larger molecules. A simplified scheme for the reaction is illustrated in Figure 3.14 and corresponds to... 

Fig ure 4-16. Trickle-bed (tubular reactor) for hydrodesulfurization. (Source J. M. Smith, Chemical Engineering Kinetics, 3rd ed., McGraw-Hill, Inc., 1981.)... 

Although desulfurization is a process, which has been in use in the oil industry for many years, renewed research has recently been started, aimed at improving the efficiency of the process. Envii onmental pressure and legislation to further reduce Sulfur levels in the various fuels has forced process development to place an increased emphasis on hydrodesulfurization (HDS). For a clear comprehension of the process kinetics involved in HDS, a detailed analyses of all the organosulfur compounds clarifying the desulfurization chemistry is a prerequisite. The reactivities of the Sulfur-containing structures present in middle distillates decrease sharply in the sequence thiols sulfides thiophenes benzothiophenes dibenzothio-phenes (32). However, in addition, within the various families the reactivities of the Substituted species are different. 

Here we illustrate how to use kinetic data to establish a power rate law, and how to derive rate constants, equilibrium constants of adsorption and even heats of adsorption when a kinetic model is available. We use the catalytic hydrodesulfurization of thiophene over a sulfidic nickel-promoted M0S2 catalyst as an example ... 

Studies of the Kinetics and Mechanisms of Ammonia Synthesis and Hydrodesulfurization on Metal Single-Crystal Surfaces... 

Kinetics over the Mo(lOO) Crystal Surface. We have studied the hydrodesulfurization of thiophene over the initially clean Mo(lOO) single crystal surface in the temperature range 520K - 690K and at reactant pressures of 100 Torr < P(H ) 800 Torr and 0.1 Torr P(Th) < 10 Torr. Under these conditions the reaction is catalyzed at a constant rate for a period of approximately one hour after which the rate begins to decrease with time. The rates reported here are all initial rates of reaction calculated from data collected in the period over which they remain constant. 

Whitehurst, Isoda, and Mochida write about catalytic hydrodesulfurization of fossil fuels, one of the important applications of catalysis for environmental protection. They focus on the relatively unreactive substituted di-benzothiophenes, the most difficult to convert organosulfur compounds, which now must be removed if fuels are to meet the stringent emerging standards for sulfur content. On the basis of an in-depth examination of the reaction networks, kinetics, and mechanisms of hydrodesulfurization of these compounds, the authors draw conclusions that are important for catalyst and process design. 

The apparent HDM reaction orders greater than unity have been attributed to the presence of more than one class of metal compounds reacting with different rates (Oleck and Sherry, 1977 Cecil et al., 1968). Just as in hydrodesulfurization, the simultaneous occurrence of several first-order reactions with different rates can lead to an apparent reaction order greater than unity (de Bruijn, 1976). Wei and Hung (1980) theoretically demonstrated conditions whereby two first-order reactions give rise to apparent second-order kinetics. 

The structural differences between the various sulfur-containing molecules make it impractical to have a single rate expression applicable to all reactions in hydrodesulfurization. Each sulfur-containing molecule has its own hydrogenolysis kinetics that is usually complex because several successive equilibrium stages are involved and these are often controlled by internal diffusion limitations during refining. 

Nevertheless, the development of general kinetic data for the hydrodesulfurization of different feedstocks is complicated by the presence of a large number of sulfur compounds each of which may react at a different rate because of structural differences as well as differences in molecular weight. This may be reflected in the appearance of a complicated kinetic picture for hydrodesulfurization in which the kinetics is not, apparently, first order (Scott and Bridge, 1971). The overall desulfurization reaction may be satisfied by a second-order kinetic expression when it can, in fact, also be considered as two competing first-order reactions. These reactions are (1) the removal of nonasphaltene sulfur and (2) the removal of asphaltene sulfur. It is the sum of these reactions that gives the second-order kinetic relationship. 

Each of the three approaches has been used to describe hydrodesulfurization of residua under a variety of conditions with varying degrees of success, but it does appear that pseudo-second-order kinetics are favored. In this particular treatment, the rate of hydrodesulfurization is expressed by a simple second-order equation ... 

Application of the second-order rate equation to the hydrodesulfurization process has been advocated because of its simplicity and use for extrapolating and interpolating hydrodesulfurization data over a wide variety of conditions. However, while the hydrodesulfurization process may appear to exhibit second-order kinetics at temperatures near 395°C (745°F), at other temperatures the data (assuming second-order kinetics) does not give a linear relationship (Figure 4-9) (Ozaki et ah, 1963). 

In spite of all of the work, the kinetics and mechanism of alkyl-substituted dibenzothiophene, where the sulfur atom may be sterically hindered, are not well understood and these compounds are in general very refractory to hydrodesulfurization. Other factors that influence the desulfurization process such as catalyst inhibition or deactivation by hydrogen sulfide, the effect of nitrogen compounds, and the effect of various solvents need to be studied in order to obtain a comprehensive model that is independent of the type of model compound or feedstock used. 

Effect of Reaction Time and Temperature. The amount of catalyst (10 g of Co-Mo-Al in 100 g of coal liquids) and the initial hydrogen partial pressure (2000 psig) determined above were used to study the effect of reaction time and temperature. Hydrodesulfurization and hydrogen consumption kinetics were determined, as outlined in the following paragraphs. 

A first order kinetic law was used to compute the hydrogenation, isomerization and hydrodesulfurization rate coefficients in mol/kg/h. 

A summary of reactor models used by various authors to interpret trickle-bed reactor data mainly from liquid-limiting petroleum hydrodesulfurization reactions (19-21) is given in Table I of reference (37). These models are based upon i) plug-flow of the liquid-phase, ii) the apparent rate of reaction is controlled by either internal diffusion or intrinsic kinetics, iii) the reactor operates isothermally, and iv) the intrinsic reaction rate is first-order with respect to the nonvolatile liquid-limiting reactant. Model 4 in this table accounts for both incomplete external and internal catalyst wetting by introduction of the effectiveness factor r)Tg developed especially for this situation (37 ). 

For most reaction systems, the intrinsic kinetic rate can be expressed either by a power-law expression or by the Langmuir-Hinshelwood model. The intrinsic kinetics should include both the detailed mechanism of the reaction and the kinetic expression and heat of reaction associated with each step of the mechanism. For catalytic reactions, a knowledge of catalyst deactivation is essential. Film and penetration models for describing the mechanism of gas-liquid and gas-liquid-solid reactions are discussed in Chap. 2. A few models for catalyst deactivation during the hydrodesulfurization process are briefly discussed in Chap. 4. 

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