Desulfurization of resids

Consideration of Catalyst Pore Structure and Asphaltenic Sulfur in the Desulfurization of Resids... 

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. 

Gulf resid hydrodesulfurization process a process for the desulfurization of heavy feedstocks to produce low-sulfur fuel oils or catalytic cracking (q.v.) feedstocks. 

Gulf Resid A process for desulfurizing petroleum residues, developed by Gulf Oil Corporation. Speight, J.G., The Desulfurization of Heavy Oils and Residua, Marcel Dekker, New York, 1981, 175. 

The role of resid desulfurization units introduced into Japan in the latter half of the 1960s has changed to the feed pretreatment for resid fluid catalytic cracking (RFCC) and the mild resid hydroconversion as well as the desulfurization of heavy oil since the oil crises. 

Resid hydrotreating catalysts, developed in 1987 have been used together with a scale-and iron-removing catalyst for (1) the cracking and desulfurization of atmospheric residue, (2) the pretreatment of RFCC and (3) the cracking of vacuum residue. Approximately 7000 tons of industrial catalysts have been used in commercial units so far. 

In the past few years the residue hydrodesulfiirization process has gone through a number of changes. Deeper desulfurization and more conversion to mid-distillate have become a primary target for several units. At the same time, heavier residues are being processed. To address these and other questions, Nippon Ketjen has developed a new series of resid catalysts, viz. the KFR series. The two most common modes of resid hydroprocessing applied on commercial scale, are illustrated with pilot plant test data and data from commercial units. 

The operating conditions, which are easy to manage, make the application of biological processes for the desulfurization of middle distillates advantageous. The process involving the recovery of the bacterial solution does not require the addition of hydrogen or any other consumable. One factor impeding application in fuel cell APUs, however, is the insufficient activity of bacterial strains known to us today. The residence times required would result in unrealistic reactor dimensions. 

A novel Double Draw-Off (DDO) ciystallizer has been designed in order to improve the particle size distribution in the precipitation of CaS03 V 20 simulated Flue Gas Desulfurization (FGD) liquor. The effects of DDO ratio and residence time on the mean particle size were studied. Industrial conditions were maintained in all experiments as far as practical. Significant improvement in mean particle size was achieved. The performance of an actual industrial DDO ciystallizer (DuPont) for gypsum ciystallization was reported. 

Fig. 36. Removal of byproduct H2S between stages increases HDS reactivity in sequential reactors (second-stage conditions 360DC, 2.9 MPa, 30 min residence time). ( ) Desulfurization performance in normally sequenced stages (O) desulfurization performance when H2S is removed between stages. Zero time indicates product composition after the first stage at 360°C, 2.9 MPa, and 30 min residence time. Figure modified and reproduced with permission from Ref. 14. Copyright 1994 American Chemical Society.

Table II compares two atmospheric resids, West Coast and Kuwait, in a traditional manner. The obvious differences include sulfur, nitrogen, asphaltenes, total metals and mid-boiling point. Apart from sulfur content, one might surmise a greater catalyst demand by the West Coast feedstock in that the boxed values suggest heavy coke laydown and metals deposition. Neither of the sulfur values is boxed because there is no indication as to (1) what fraction of the sulfur is refractory or "hard" sulfur, nor (2) the degree of desulfurization to be achieved.

As we can see from Table III, 86% of the sulfur contained in all of the oxidation products resides in the soluble oxidation products even though this product fraction only represents 52% by weight of the combined oxidation products. This indicates that organic sulfur compounds have been preferentially depolymerized from the coal matrix and that this oxidative procedure may be a possible desulfurization process. 

Figure 42 shows that the availability of calcium for desulfurization in coal combustion in a bubbling fluidized bed decreases with increasing particle diameter and is less than 20% for particles larger than 1 mm. On the other hand, fly ash collected from flue gas shows a calcium availability of not more than 10%, possibly due to its short residence time in the combustor. 

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 sulfur regulation (500ppmw sulfur) in the U.S. were to increase process severity of HDS, increase catalysts-to-fuel ratio, increase residence time, and enhance... 

Every year many new catalysts become available for a wide variety of applications. Because it is impossible to discuss all these catalysts, we will limit the discussions to a summary of new grades introduced in the last five years and review only the most important developments for the various application segments, being resid upgrading, hydrocracking, FCC pretreatment, middistillate desulfurization and deep hydrogenation. Table 6 summarizes new catalysts introduced in the last 5 years and still being sold in 1995 [35]. Each year 15-20 new catalysts are introduced in the market. Akzo Nobel, Chevron and Criterion appeared to be the most active companies. 

The experiments were conducted on a Kuwait atmospheric resid with characteristics as given in Table 1. The catalysts tested were commercial resid desulfurization catalysts in the shape of 1/32" cylindrical extrudates. Important properties of the catalysts have been listed in Table 2. 

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