Coal dissolution process

Direct-Liquefaction Kinetics All direct-liquefac tion processes consist of three basic steps (1) coal slurrying in a vehicle solvent, (2) coal dissolution under high pressure and temperature, and (3) transfer of hydrogen to the dissolved coal. However, the specific reac tion pathways and associated kinetics are not known in detail. Overall reaction schemes and semiempirical relationships have been generated by the individual process developers, but apphcations are process specific and limited to the range of the specific data bases. More extensive research into liquefaction kinetics has been conducted on the laboratory scale, and these results are discussed below. 

Since more than one of these dissolution processes might occur in the coal extraction experiments, it is necessary to allow for concurrent chemical reactions when constructing a rate equation. Since reactions are either first or second order, a kinetic expression having concurrent reactions of first and second order must be derived. 

Dr. Heredy We agree that cracking probably occurs, and the resulting radicals react with phenanthrene. However, we are not sure how important hydrogen availability is for the dissolution process although hydrogen exchange does occur with coal, it occurs only to a small extent or not at all in the dissolution of model compounds. 

The following conclusions do not imply that the C1-C4 gas yield, C5-500°F liquid yield, 500°F+ conversion, and 650°F+ conversion have been optimized for the TSL process which contains a first stage coal dissolution (SRC-I or SCT) followed by LC-Finer hydrotreating. The conclusions are directly applicable only to the LC-Finer. 

For the coal studied here, a bituminous Western Kentucky 9/14 coal, dissolution of the coal has been shown to occur very rapidly, requiring less than 30 minutes to liquefy most of the coal ( 90 percent). However, a relatively long reaction time (120 min.) is required to reduce its sulfur level low enough to meet even the current standards (1). A new short residence time two-stage SRC type process has been suggested by Auburn University (2 ) for solvent refining this coal. This process has been shown to have the potential of producing a low-sulfur solid SRC product that meets the proposed NSPS. It involves the dissolution of the coal... 

The objective of the present work is to evaluate the effect of a wide range of process or reaction variables— reaction temperature, hydrogen partial pressure, catalyst loading, and reaction time—on hydrodesulfurization and hydrogenation of filtered liquid product (coal-derived liquid) obtained from the coal dissolution stage in the presence of a commercial presulfided Co-Mo-Al catalyst. The selectivity for desulfurization over hydrogenation (Se) is used to rate the effectiveness of the above mentioned process variables. Se is defined as the fraction of sulfur removal per unit (g) of hydrogen consumed, that is,... 

In the two-step SRC process the purpose of the first step is to dissolve the coal at short contact times. The second step, using more severe conditions, might be used to regenerate solvent quality or, if necessary, to reduce the sulfur content of the product. A single coal dissolution step might find practical application if solvent were available from a nonprocess source. 

A major source of emissions from the coal dissolution and liquefaction operation is the atmospheric vent on the slurry mix tank. The slurry mix tank is used for mixing feed coal and recycle solvent. Gases dissolved in the recycle solvent stream under pressure will flash from the solvent as it enters the unpressurized slurry mix tank. These gases can contain hazardous volatile organics and acid gases. Control techniques proposed for this source include scrubbing, incineration, or venting to the combustion air supply for either a power plant or a process heater. 

The preheater is essentially a plug flow reactor (L/D > 100) where the coal-solvent slurry and hydrogen gas are preheated to the liquefaction temperature. Extensive studies have been carried out to understand the exact nature of the processes, taking place in the preheater (2,11,13-16). Considerable work has also oeen performed to evaluate the dissolution process and its effect on thermal hydraulics of large-scale preheaters (13-16). 

The slurry in their case was treated as a homogeneous phase and the solid distribution was not taken into account. The model parameters were estimated from the correlations proposed for the case of no solid suspension (two-phase system). The coal dissolution, hydrogenation and hydrodesulfurization were considered as the key reactions, in the model. Also, the effect of mass transfer on the liquefaction process has been investigated. The model predictions were found to be in good agreement with the experimental... 

In contrast to the SRC-II Process, where coal dissolution and coal and solvent hydrocracking are achieved non-selectively in a single unit under largely non-optimal thermal conditions, the TSL... 

Data for the kinetics of coal liquefaction have been published in the literature (1-11). A review of the reported studies has recently been given by Oblad (12). The reported data were mostly obtained in bench-scale reactors. Guin et al. (7) studied the mechanism of coal particle dissolution, whereas Neavel (7), Kang et al. (8), and Gleim (10) examined the role of solvent on coal liquefaction. Tarrer et al. (9) examined the effects of coal minerals on reaction rates during coal liquefaction, whereas Whitehurst and Mitchell (11) studied the short contact time coal liquefaction process. It is believed that hydrogen donor solvent plays an important role in the coal liquefaction process. The reaction paths in a donor solvent coal liquefaction process have been reviewed by Squires (6). The reported studies examined both thermal and catalytic liquefaction processes. So far, however, very little effort has been made to present a detailed kinetic model for the intrinsic kinetics of coal liquefaction. 

SCT-SRC could be further processed at temperatures above that of dissolution to produce a clean solid fuel of reduced sulfur content. Char formation tendency would be lowered by prior removal of mineral matter and undissolved coal. At higher temperatures, desulfurization would proceed rapidly light gas formation might be minimized by keeping the time very short. Hydrogen consumption would be minimized because aromatic-hydroaromatic equilibria favor aromatics as temperatures increase. 

Table VII shows the estimated yields from an SCT-SRC plus hydrotreating scheme along with published yields from SRC-I (5), SRC-II (6) and H-coal Syncrude (7) processes. The yields for the SCT dissolution operation at Wilsonville (8) are also included...

Although the comparisons are by no means exact, a process based on short-contact time dissolution and catalytic upgrading would appear to have potential for significantly higher yields of high quality liquids from coal. 

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