Coal liquid aromaticity

Benzene (CeHg) is the simplest aromatic hydrocarbon and by far the most widely used one. Before 1940, the main source of benzene and substituted benzene was coal tar. Currently, it is mainly obtained from catalytic reforming. Other sources are pyrolysis gasolines and coal liquids. 

Girgis, M. J., and Gates, B. C., Catalytic Hydroprocessing of Simulated Heavy Coal Liquids. 2. Reaction Networks of Aromatic Hydrocarbons and Sulfur and Oxygen Heterocyclic Compounds. Ind. Eng. Chem. Res, 1994. 33 pp. 2301-2313. 

The analysis of Wyodak recycle solvent by SEC-GC-MS shows that certain distribution order exists for species in coal liquids with respect to their size and degree of isomerization. The alkanes increase in chain lengths without any appreciable degree of isomerization, except for some biological markers such as pristane and phytane. Phenols and aromatics vary in size and extent of isomerization which causes the liquid to contain a large number of species. 

Phenols are a major chemical lump present in coal liquids. Phenols have basically one or more aromatic ring structures with alkyl substituents. Methyl, ethyl and propyl are the most common alkyl substituents. The smallest specie is the one with a hydroxyl group attached to a benzene ring. Addition of a methyl group produces three isomers - o-, m-, and p-cresols. It appears that all three are present in more or less same proportion. The number of possible isomers increases as the possible number and size of alkyl substituents increases. It is expected that higher... 

The species which are unknown and have not been identified as one of the major chemical lump such as alkanes, phenols and aromatics are lumped together as unidentified. However, the species in this lump include saturated and unsaturated cycloalkanes with or without side chains, which resembles the naphthenes, a petroleum refinery product group. A number of well known species in coal liquid are not mentioned in this lumping scheme. Such as heterocyclic compounds with sulfur, nitrogen or oxygen as the heteroatom, and other heteroatora containing species. Some of these compounds appear with aromatics (e.g. thiophenes, quinolines) and with phenols (e.g. aromatic amines), and most of them are lumped with the unidentified species lump. 

Coal Liquids. The two coal liquids contain about the same amount of material boiling below 470°F, very little saturates, and substantial amounts of aromatics, mainly di- + triaromatics (Table II). The liquids from the Big Horn coal, however, contain more aromatics and less resins, asphaltenes, and benzene insolubles than the liquids from Pittsburgh Seam coal. This is not surprising considering the fact that higher rank... 

Shale Oils. As indicated by the data in Table II, only cut I of the shale oils contains a considerable amount of material boiling below 470°F, as would be expected from the low boiling range of this fraction (Table V). This distillate was fractionated into saturates, aromatics, and olefins by preparative FIA techniques, but a GLC analysis of these fractions proved unfruitful for the same problems mentioned above for the coal liquids. 

Comparison of Fuels. The separation data of Table II show immediately some gross differences and similarities among the various fuels analyzed. Both coal liquids have a considerable amount of low boiling material which is made up of saturates and aromatics in about equal... 

Very commonly, however, the sample of interest is not a pure compound, but is a complex mixture such as a coal liquid. As a result, a specific structure determination for each molecular type is not practical, although it is possible to determine an average chemical structure. Features which may be determined include the hydrogen distribution between saturate, benzylic, olefinic, and aromatic sites. The carbon distribution is usually split into saturate, heterosubstituted saturate, aromatic + olefinic, carboxyl, and carbonyl types. More details are possible, but depend greatly on the nature of the sample, and what information is desired. 

Most coal liquids are composed of similar major chemical species, which may differ in exact composition. Liquid sulfur dioxide can be used to extract all the aromatic species of the coal liquid, free of saturated hydrocarbons and ash percursors. After removing the SO2 by degassing, distillation under reduced pressure can yield all the phenols and aromatic from the S02-solu-bles of the coal liquid. The residue, which is similar to GPC -fraction 2 of the S02 solubles, can be called coal asphaltenes. 

Table III shows elemental composition of typical sour petroleum, coal syncrudes or shale oils. Compared with typical sour petroleum, the coal syncrude is lower in sulfur content but significantly higher in nitrogen. Compared with shale oil, coal syncrude is lower boiling and contains only about one half the nitrogen. A major difference between the two liquids is the highly aromatic structure of coal liquids and the absence of long paraffinic structures. Shale oil is more aromatic than petroleum but significantly less aromatic than coal liquids. This is mirrored by the hydrogen contents which were shown in Table I.

On this basis, conversion is limited by coal structure. And in terms of the conventional homolytic sclsslon/H-capping view of conversion, increased yields of coal liquids are therefore obtainable only through increases in conversion temperature or residence time. Unfortunately, increases in the thermal severity of the process result in products reflecting the rise of dealkylation and aromatization reactions at higher temperatures. Thus increased product yields are brought about at a considerable cost to product quality (5). 

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