Liquefaction, coal mechanisms

In outline of what follows we will begin by brief reference to previous work on coal liquefaction. The present approach will then be motivated from considerations of coal structure and hydro-gen-donor activity. A theoretical section follows in the form of a pericyclic hypothesis for the coal liquefaction mechanism, with focus on the hydrogen transfer step. Experiments suggested by the theory are then discussed, with presentation of preliminary results for hydrogen transfer among model substrates as well as for the liquefaction of an Illinois No. 6 coal to hexane-, benzene-, and pyridine-solubles by selected hydrogen donors. 

The present approach to the coal liquefaction mechanism evolved from contemporary knowledge of coal structure (e.g. 11,... 

Szladow, A. J. "Some Aspects of the Mechanism and Kinetics of Coal Liquefaction , Ph.D. Thesis, Pennsylvania State University, 1979, 172 pp. 

A considerable effort has been emended in the past few years by many researchers in attempts to better understand the mechanism by which coal is liquefied. From this work has emerged the concept of short residence time coal liquefaction which promises potential process advantages, small reactor, minimum hydrogen flow, and the efficient utilization of hydrogen for a particular product slate. 

Various mechanisms and kinetics of coal liquefaction have been proposed and examined by many investiga tors(l,2,4-8). As a general kinetic model of coal lique-action, scheme 1 was assumed. The reaction rate of every reaction step in the scheme assumed to be first order with respect to reacting species and dissolved hydrogen. A few typical cases of a general kinetic model and the general characteristics for their cases are illustrated on Table 3. When compared these typical figures, the curves are apparently different in shape. 

The effects of various reaction conditions on the reaction rate and the mechanism of coal liquefaction were investigated.Conclusions are summarized as follows ... 

Under the same reaction conditions, the reaction rate are depend on the mechanism of coal liquefaction and kinds coal and catalyst. The reaction rate is in the following order Morwell> Bukit... 

In the present study, the liquefaction activities of pyrene, its derivatives, and decacyclene with coals of several ranks are studied to ascertain the previous ideas of liquefaction mechanism and to develop novel liquefaction process under atmospheric pressure. The coals used in the present study are non-fusible or fusible at relatively high temperature, and then gave small liquefaction yield with pyrene of a non-solvoly-tic solvent at 370°C. 

In a previous paper ( 5), the authors described the liquefaction mechanism according to the properties of the coal and the solvent. The coal was classified into two categories. 

The liquefaction mechanism was discussed by distinguishing the fusible coal from non-fusible one. The importance of solvolytic hydrogen transfer is pointed for the liquefaction of non-fusible coal under atmospheric pressure. 

The object of this paper is to draw attention to the possible importance of concerted molecular reactions, of the type termed pericyclic by Woodward and Hoffman (1), in the mechanism of coal liquefaction. 

The preceding experiments offer preliminary support to our notion that pericyclic pathways might be intimately involved in the mechanism of coal liquefaction. More specifically, the results indicate that pericyclic group transfer reactions constitute a plausible pathway for the transfer of hydrogen from donor solvents to coal during liquefaction. 

Fully-deuterated xetralin was used to study the mechanisms of coal liquefaction. Experiments were conducted with xetralin-di2, deuterium g s and bituminous coal at 400°C and at 15.2-20.7 MPa. The recovered solvent and solvent-fractionated coal products were analyzed for total deuterium content and for deuterium content in each structural position. 

In an earlier presentation, we reported on a deuterium tracer method for investigating the mechanisms of coal liquefaction Q). 

Much of the research pursued by the authors of this paper and by their associates has involved studies of the catalytic hydrogenation of coals in the absence of solvent. The technique has been used to elucidate the mechanisms of catalytic coal liquefaction and to provide simultaneously some insight into the structure of coals. Peter Given was directly instrumental in providing the incentive for this research which has extended since 1983. Previous findings were disseminated through several publications (4-8. In this paper, some of the earlier data have been collated with more recent results (9) to provide an account of the relevance of these studies to the two-component concept. 

In preceding sections, fundamental coal chemistry, liquefaction mechanisms, solvent and catalyst characteristics were summarized briefly. In the following three sections, the roles and improvements in solvents and catalysts in multistage liquefaction processes are reviewed in more detail on the basis of recent progress in this area. 

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 chemistry of coal liquefaction is not very well understood, even after more than two decades of research into the kinetics and mechanism of the process. There have been a number of models for conversion proposed, most of them focused on the several liquefaction products, including preasphaltenes, asphaltenes, oils, and gases. A survey of some of the models has been presented (1 ), and a common feature among them is the multiplicity of paths connecting all of the components. 

At the present time, few, if any, details of chemical reaction mechanisms in coal conversion are known with certainty. This situation is particularly distressing in the areas of coal liquefaction and pyrolysis where chemical kinetics may strongly influence process efficiency and product quality. To improve this situation, in recent years a number of research groups have been performing chemical studies of coal and "model" compound reactions. 

The potential importance of reactions involving ions or ion pairs in coal and model compound reactions has been emphasized by Ross and co-workers (42) as well as by Brower (43). For many types of reactions there exists considerable debate concerning reactive intermediates and mechanism. However, in the case of water formation, which is known to be rapid during coal liquefaction under relatively mild conditions and appears to occur in certain model compound reactions (15), it is difficult to construct plausible pathways without postulating ionic intermediates (although these intermediates may reside on solid surfaces). 

Shinn (1982) developed a mechanically agitated slurry reactor with induction heaters for coal liquefaction. While the induction heaters required large power input, they allowed the slurry to heat up to 400-450°C in few minutes, thus cutting down the heat-up period. In the use of such reactors, the effects of induction heating on the metal degradation and failure need to be carefully considered. Except for the induction heating system, the rest of the reactor was a conventional slurry reactor. The concept of induction heating is more practical for smaller-size reactors. 

When the catalyst is available in a small amount, a microreactor assembly is often used (Miller, 1987). This is a simple T-type reactor heated by a fluidized sand bath. The mixing is provided by mechanical agitation that shakes the reactor up and down within the fluidized bed. Because of the small amount of slurry, and an effective heat transfer in the fluidized sand bath, the heat-up period in such a reactor is small. The nature of mechanical agitation is, however, energy-efficient. The reactor provides only a small sample for the product analysis, which makes the usefulness of the reactor for detailed kinetic measurements somewhat limited. The reactor has been extensively used for laboratory catalyst screening tests in coal liquefaction. 

In certain cases, such as the separation of PAHs obtained from a coal liquefaction process, using reversed-phase HPLC is complicated as sample preparation is elaborate. This is due in large part to the fact that most complex fuel-related materials contain compounds that are not usually soluble in acetonitrile, the solvent of choice in reversed-phase HPLC. Here, NPC, which employs a variety of solvents, offers an alternative to the analysis of such samples. Separation of five well-studied coal liquefaction process stream samples was achieved and 19 isomers were resolved when NPC was used [33]. The method employed a tetrachlorophthalimidopropyl-modified silica column (TCPP) with a charge-transfer mechanism. 

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