Coal liquefaction catalyst deactivation

The second approach depends heavily on obtaining very specific coals, which may exist in only limited supplies. Carbonates and chlorides have been found to be easily extracted by weak acids, such as acetic acid, without harming catalysts such as FeS or Ni-MoS, although regeneration and resulfiding may be necessary because some of the sulfur may be replaced by oxygen during liquefaction. Catalyst deactivation by minerals deposition will not be of concern for coals extracted in this way. 

The results have shown that spinning/falling basket autoclaves can be used effectively for gathering data on coal hydroliquefaction, a single contact being representative of steady state conditions. As with other types of reactors for coal liquefaction, the catalysts were deactivated to a constant activity but the rate of deactivation was much more rapid in tiie autoclaves. 

Catalysts in coal liquefaction are used in moving-bed, ebulating-bed, and fixed-bed processes. Disposable iron catalysts must be used in moving beds. More expensive Co-Mo and Ni-Mo catalysts are used in either ebulating or fixed beds, and catalyst deactivation rates and ultimate lifetime are of concern (80, 81). In ebulating beds, a small portion of fresh catalyst is continuously fed to balance the catalyst being purged. 

Catalysts for coal liquefaction require specific properties. Catalysts of higher hydrogenation activity, supported on nonpolar supports, such as tita-nia, carbon, and Ca-modified alumina, are reasonable for the second stage of upgrading, because crude coal liquids contain heavy polar and/or basic polyaromatics, which tend to adsorb strongly on the catalyst surface, leading to coke formation and catalyst deactivation. High dispersion of the catalytic species on the support is very essential in this instance. The catalyst/support interactions need to be better understood. It has been reported that such interactions lead to chemical activation of the substrate 127). This is discussed in more detail in Section XIII. 

The difficulty in the recovery of catalysts from unreacted coal and minerals and the poor regenerability of used catalysts forces one to use disposable catalysts, especially in the primary stage. This increases the cost of coal liquefaction considerably. This section reviews the mechanism of catalyst deactivation, design of recoverable catalysts in the primary stage, and catalyst deactivation in the secondary stage. 

The recovery, regeneration, and repeated reuse of the active catalyst are of prime importance in substantially reducing the overall cost of coal liquefaction. The used catalysts usually remain in the bottoms products, which consist of nondistillable asphaltenes, preasphaltenes, unreacted coal, and minerals. The asphaltenes and preasphaltenes can be recycled with the catalyst in bottoms recycle processes. However, unreacted coal and minerals, if present in the recycle, dilute the catalyst and limit the amount of allowable bottoms recycle because they unnecessarily increase the slurry viscosity and corrosion problems. Hence, these useless components should be removed or at least reduced in concentration. If the catalyst is deactivated, reactivation becomes necessary before reuse. Thus, the design of means for catalyst regeneration and recycle is necessary for an effective coal liquefaction process. Several approaches to achieving these goals are discussed below. 

Catalysts currently employed in process development units for coal liquefaction are hydroprocessing catalysts developed for petroleum refining (5l6). They are composed of combinations of Mo or W with Co, Ni or other promoters dispersed on alumina or silica-alumina supports. When used in liquefaction, these catalysts deactivate rapidly f6-9i causing decreases in product yield and quality and problems with process operability. Thus the... 

The heteroatom reactants do not themselves deactivate the catalyst the catalyst deactivates due to coke formation from hydrocarbons and metal deposition from the mineral in coal. The commercial HT process cannot be easily carried out on a bench scale, because of materials handling and pressure problems however, the process is carried out on a demonstration scale at the Advanced Coal Liquefaction Research and Development Facility at Wilsonville, AL. Small portions of the catalyst are removed at various deactivation levels, quantified as the weight of product per weight of catalyBt, with the metals and coke deposits on the removed solid being characterized. 

The multistage liquefaction process (Figure 19.17) involves the liquefaction of crushed, dried coal which is slurried with an aromatic recycle solvent and the whole is then charged to a reactor at 415°C-440°C (VSOT-STST) at approximately 2000 psi. An expanded bed reactor system is employed to circumvent the problems that arise through reactor plugging, catalyst deactivation, and inequitable liquid distribution. 

Exxon Donor Solvent Minerals in coal in liquefaction reactor CoO—M0O3/AI2O3 in separate hydrogenation reactor 450 140 Tetralin in liquefaction reactor. Recycled after hydrogenation in separate reactor Further hydrogenation of product liquids in separate reactor— catalyst deactivation slow. Typical product yields 0-3 to 0-4 te liquid/te coal feed... 

Studies by CasteUo et al. [46,47] demonstrated that the composition, origin, and stability of natural and synthetic aluminosilicates and allophanes can be determined by LA-FT-ICR-MS. In fact, LDI-MS provides a method to perform semiquantitative measurements of the ALSi ratio in solid materials [47]. Greenwood et al. [39] used LDI-MS to detect low molecular weight organic compounds of m/z 200-300 range, which apparently cause catalytic deactivation, on the surface of a zeolite catalyst used for coal liquefaction. 

Dispersed catalysts are defined as heterogeneous catalysts flowing alrnig with the reactants in a reactor system. The residence time of catalyst and reactants are thus equivalent and consequently, a continuous renewal of the catalyst-reactant interface is afforded. They demonstrate their usefulness in case of competitive fast deactivation or when accessibility of reactants to catalyst surface is hampered by feed characteristics. This kind of situation typically correi nds to heavy feeds processing, and effectively, the use of dispersed catalysts is practically limited to coal liquefaction and petroleum residues conversion. 

Coal and its derived liquids contain a number of catalyst poisons or poison precursors, which inevitably and severely deactivate the catalysts during the liquefaction process. The catalysts are deactivated via three major routes... 

A question of considerable interest in coal hydroliquefaction chemistry is the amount and nature of "organically bound metals in the coal. One reason for this interest is the observation that when supported metal direct conversion catalysts are used in liquefaction reactors, a primary mode of deactivation is metals deposition Q, 2). In particular, recent work at the Pittsburgh Energy Technology Center (PETC) (4,5) and elsewhere (3) has indicated very high levels of titanium deposition on supported Co Mo catalysts used in the fixed bed continuous reactor system. It has been suggested that the culprits in such deposition are soluble metal species (6 9) The analyses of a Western Kentucky (Homestead) hvBb feed coal and of material deposited between the catalyst pellets in the fixed bed reactor at PETC (4) are shown in Table I. 

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