Influence of mixing on short contact time coal liquefaction

The influence of mixing on the liquefaction behaviour of coal at short contact times (? 20 min) has been studied for bituminous coals from Powhatan (USA) and Liddell (Australia). Experiments were carried out in 1.0 and 0.3 dm³ reactors, respectively, both of which had facilities for varying stirrer speed and for rapid injection of coal and solvent. Results at 450 °C with the Powhatan coal revealed that at a low agitation rate (1500 rev min^?1) the yields of oil were negative. At a higher stirrer speed (2500 rev min^?1) the agitation was sufficient to eliminate this oil deficiency. In the Liddell experiments, the conversion rate at a stirrer speed of 865 rev min^?1 was significantly lower than at 1860 and 2380 rev min^?1. Rate coefficients calculated for stirrer speeds of 865 and 1860 rev min^?1 were found to differ by more than a factor of two. These data illustrate that the degree of agitation can have a significant bearing on the rate of total conversion of coal to soluble products and on the yield of oil, suggesting that mass transfer limitations can occur at the coal-solvent interface.     

Simulation of chemical rate processes in short contact time coal liquefaction

Conversions and product distributions from the thermal dissolution of Liddell coal in the presence of tetralin have been simulated by a system of second-order rate equations. Satisfactory correlation with experimental data is obtained over the range of temperatures studied, i.e. 380–420 °C.     

Effects of coal cleaning on the short contact time liquefaction of Illinois No. 6 coal

This study was carried out to determine the effect of coal cleaning by oil agglomeration and sink-float methods on yields from short contact time liquefaction of Illinois No. 6 coal. The runs were made in a continuous unit using SRC-II distillates as process solvent. Measured yields included hydrogen (consumption), hydrocarbon gas, distillate oil, SRC (the pyridine-soluble portion of the residue) and insoluble organic matter, the pyridine-insoluble organic residue. The solubility of product SRC in hexane, toluene and pyridine was also determined. The principal finding was that coal cleaning by density methods reduced the yield of IOM obtained in subsequent liquefaction and this is attributed to the removal of inert components from the feed coal. In addition, cleaning which significantly reduced pyrite content of the feed coal also reduced the yield of distillate oil and tended to give a less soluble SRC during liquefaction. Deep cleaning by gravity methods gave the lowest IOM, but reduced pyrite content to the point where distillate oil was consumed rather than produced. Oil agglomeration reduced total ash to 50% of that in the run-of-mine coal, but left the pyrite level in the coal high. The relevance of these results to two-stage liquefaction is discussed.     

The roles of vehicle and gaseous hydrogen during short contact time coal liquefaction

The roles of the vehicle and gaseous hydrogen during short contact time liquefaction (SCTL) in microreactors were examined by using a number of synthetic vehicles. The importance of vehicles during SCTL was evaluated on their ability to convert coal to tetrahydrofuran (THF)-soluble products. Reaction conditions were three minutes at 425 °C following a one minute heat-up. Although the consumption of hydrogen (hydrogen donor or gaseous hydrogen) by reactive coal fragments is minimal during the early stages of liquefaction (as in SCTL reactions), hydrogen donors were important and could be ranked according to the extent of conversion to THF-soluble products (tetrahydroquinoline > hydrogenated pyrene > dihydroanthracene = dihydrophenanthrene > tetralin). The importance of gaseous hydrogen was also studied. Gaseous hydrogen was needed if either quality or quantity of hydrogen donor was not adequate in a SCTL reaction. With certain synthetic vehicles, the SCTL stage of an integrated two-stage liquefaction process could be conducted in the absence of hydrogen. Solubility properties that might enhance solubilization of coal-derived products, and therefore the extent of liquefaction, were examined by varying the concentration of certain components (m-cresol and quinoline) of the synthetic vehicle mixtures. Solubility effects were minimal.     

Nonsolvent coal hydrogenation at short contact time

Yields and properties of products on hydrogenation of Japanese and Australian coals have been studied using an autoclave equipped with a magnedrive device at temperatures of 500–600 °C and hydrogen pressure 4.9–14.7 MPa (gauge) in the absence of solvent. Optimum contact time, at which maximum extraction yield was observed, shifted from 15 s to a few seconds with increasing reaction temperature and hydrogen pressure. The extracts derived from both coals reveal similar structural parameters.     

Short contact time liquefaction of coal using hydrogen donor solvent

Liquefaction of coals to form benzene-soluble materials was studied at 400 °C under autogenous pressure conditions using tetrahydroquinoline (THQ), tetrahydroisoquinoline (THIQ) and tetralin (TL) as the hydrogen donating solvents. THQ was the best solvent with a conversion rate of 90% within 15 min for low rank coals (< 80% C). In contrast, it took 50 min to achieve a conversion of 80% when TL was used as the solvent, although both solvents could achieve almost complete conversion of coals into quinoline-soluble material within 10 min. THQ also showed excellent activity with blended coals. Some binary solvents exhibited activities which varied with the THQ content. A 1:3 by weight mixture of THQ and petroleum pitch produced the highest conversion of 100%.     

Kinetics of thermal liquefaction of coal

A model is presented for the kinetic study of the thermal liquefaction of Belle Ayr subbituminous and Burning Star bituminous coals with anthracene oil, hydrogenated anthracene oil and hydrogenated phenanthrene. All experiments were performed in a continuous-feed, stirred tank reactor, at a temperature of 450 °C and a space time of approximately 5 to 55 min. A kinetic model which includes a reaction: coal + oil→more reactive coal, correlates the data reasonably well. This reaction explains the net consumption of anthracene oil during the initial stages of liquefaction. Such a reaction may account for a portion of the swelling of coal at low space times and the sizable increase of viscosity of reaction slurry during these initial stages of liquefaction. It is also observed that the yield of oil increases when solvents of increasing hydrogen donor capacity are used.     

Efficiency of hydrogen transferring liquefaction of a subbituminous coal using hydrogenated fluoranthene for short contact time at high temperature and low pressure

An Australian subbituminous coal (Wandoan) was effectively liquefied at 490 and 510 °C under nitrogen pressure of 2.5 MPa for 1.0–7.5 min using 1, 2, 3, 3a-tetrahydrofluoranthene (4HFL) as a hydrogen-donating solvent. The yields of oil and asphalthene could be as high as 58 and 24 wt%, respectively. The content of 4HFL was very influential on the oil yield although under appropriate liquefaction conditions, a considerable amount remained after reaction. The kinetics of the reaction and analytical study of the products and the solvent suggest consecutive as well as instantaneous depolymerization in the process. The coking or recondensation reaction was very rapid after 4HFL was consumed, confirming the efficacy of the short contact time liquefaction.     

Differences in the behaviour of US bituminous and subbituminous coals during short contact time liquefaction

The short contact time (SCT) liquefaction of Belle Ayr subbituminous coal has been compared with that of Illinois No. 6 and Pittsburgh seam bituminous coals. Each bituminous coal was highly solubilized (90 wt%, daf coal) in 3–4 min at 450 °C and 13–16 MPa hydrogen pressure. More than 80 wt% of each coal was converted to solvent-refined coal (SRC, pyridine-soluble residuum), with only small quantities of distillate oil and C₁–C₄ gas being formed. A longer reaction (up to 30 min) gave only a small increase in total conversion, but gas and distillate yields increased significantly. Iron sulphides did not appear to catalyse coal solubilization. By contrast, only 65 wt% of the Belle Ayr coal dissolved rapidly in SCT liquefaction and pyrite addition catalysed the conversion of the remaining insoluble organic matter (IOM). With an equivalent amount of pyrite present the Belle Ayr coal also gave more C₁–C₄ gas and substantially more distillate in SCT liquefaction than the bituminous coals. These differences in product distributions obtained from bituminous and subbituminous coals in SCT liquefaction can be rationalized on the basis of differences in the structures of the starting coals. However, the origin of high IOM yields with the Belle Ayr coal remains unclear.     

Kinetics of donor-vehicle coal liquefaction in a flow reactor

Development of the Exxon Donor Solvent process has necessitated studies to more clearly define the influence of liquefaction conditions on coal conversion. Process studies were conducted in a 22.7 kg/day (fifty-pound-per-day) integrated pilot plant. Illinois No. 6 coal was liquefied at temperatures from 644 to 756 K, pressures of 10 and 17 MPa, and a tenfold range of residence time. A kinetic model was developed in terms of an empirical classification of four reactive coal constituents. The reactive classes are an incorporation of concepts published in other coal-liquefaction kinetic studies. The effect of vehicle vaporization on actual residence time was used in evaluating the kinetic parameters for this study in a tubular flow reactor. The kinetic model accurately predicts distillation bottoms yields, which ranged from 41 to 72 wt % on coal. Material-balanced quadratic equations are presented for other liquefaction products.     

Kinetics of coal liquefaction during heating-up and isothermal stages

Direct liquefaction of Shenhua bituminous coal was carried out in a 500 ml autoclave with iron catalyst and coal liquefaction cycle-oil as solvent at initial hydrogen of 8.0 MPa, residence time of 0–90 min. To investigate the liquefaction kinetics, a model for heating-up and isothermal stages was developed to estimate the rate constants of both stages. In the model, the coal was divided into three parts, easy reactive part, hard reactive part and unreactive part, and four kinetic constants were used to describe the reaction mechanism. The results showed that the model is valid for both heating-up and isothermal stages of liquefaction perfectly. The rate-controlled process for coal liquefaction is the reaction of preasphaltene plus asphaltene (PAA) to oil plus gas (O + G). The upper-limiting conversion of isothermal stage was estimated by the kinetic calculation.     

Catalytic effect of metallic halide on non-solvent coal hydrogenation at short contact time

Both catalytic and non-catalytic hydrogenations of Japanese coal have been studied using an autoclave equipped with a magnedrive device at temperatures 500–600 °C and hydrogen pressure 4.9–14.7 MPa (gauge) in the absence of solvent. Gas yield decreased but extraction yield with pyridine increased markedly in the reaction using ZnCl₂ or SnCl₂ · 2H₂O as a catalyst in comparison with the non-catalytic reaction. The structural parameters of extracts derived in both reactions were determined, and showed only slight differences.     

The pyrolysis of bitumen-impregnated sandstone in short contact time reactors,I. Cyclone reactor

The concept of a short contact time, cyclone reactor has been evaluated for the pyrolysis of bitumen-impregnated sandstone from the Whiterocks and PR Spring tar sand deposits of the State of Utah. Approximately, 18 s were required for a 0.025 cm diameter bitumen-impregnated sand particle to achieve 90% conversion of the bitumen to hydrocarbon products. Thermal analysis data (TGA) was obtained as a function of particle size to determine the extent of bitumen conversion to products: hydrocarbon liquids and gases and a carbonaceous residue on the sand. The extent of the pyrolysis reaction or the conversion decreased with increased sand particle size at a fixed pyrolysis temperature and reaction time.A 400-cm diameter cyclone reactor would provide adequate residence time at pyrolysis temperatures to achieve greater than 90% conversion of the bitumen to hydrocarbon liquids and to C₁–C₄ hydrocarbon gases for sand particles having a diameter less than 0.03 cm. The cyclone reactor does not provide adequate retention time for sand particles having a diameter in excess of 0.1 cm, however, a riser-cyclone reactor would permit the processing of larger diameter particles.     

A kinetic investigation of coal liquefaction at short reaction times

Coal liquefaction kinetics have been studied at very short reaction times (less than 250 seconds) in order to emphasize the initial underlying physical and chemical processes involved. These studies were made possible by the use of a continuous flow stirred tank reactor (CSTR) which avoids the problems of slow heat up and cool down associated with the massive equipment required for running high-temperature and high-pressure liquefaction reactions. Preliminary physical (NMR and ESR) and chemical analytical results are presented on the coal liquids and reaction residues from Illinois No. 6 hv bituminous and Wyodak Black Thunder subbituminous coals.

ESR results showed that radical concentration in the solid residue changed during coal liquefaction. These changes were accompanied by changes in the NMR-derived aromaticity. The rate of decrease of organic-based radicals was different for Wyodak Black Thunder and Illinois No. 6 coals, perhaps indicating a different mechanism for the quenching of radicals in these bituminous and subbituminous coals. NMR spectra of the liquid products indicated that the initially produced material was relatively aromatic, and that subsequent products had lower aromatic content. This is consistent with secondary hydrogenation of the primary liquefaction products. Finally, the total oxygen contents of the coal residues decreased gradually during the first three minutes of coal liquefaction at 390°C. A corresponding decrease in the hydroxyl content of these residues was also noted.     

Study on the operating variables of sulcis coal hydroliquefaction

The hydroliquefaction of a high sulphur and bituminous Italian coal (Sulcis, Sardinia) was studied batchwise in a 1.5 dm³ autoclave to assess the influence of coal comminution (d_p = 45?664 μm), coal/solvent (c = 0.22 to 0.37 g coal/g solvent) and catalyst/coal (Y = 0?15 mg catalyst/g coal) ratios, and reaction time (t = 60?120 min) on the overall coal conversion (η_h), yields of oil (η_o) and asphaltene (η_a) and hydrodesulphurization (η_hs) by means of a composite design technique. Significant effects were observed on η_h by d_p, on η_o and η_a by Y, and on η_hs by Y and t, by means of an F-test on the basis of Yates's method. The variables were then fitted to a quadratic expression with an overall mean standard error of about 10%.The optimal range of the operating variables under the experimental conditions (i.e., 405°C, 150 bar) was derived by statistical treatment of the hydroliquefaction yield data and found to be d_p = 0.13 mm (100–150 mesh), c = 0.3 g coal/g solvent, Y = 13–15 mg catalyst/g coal and t = 100?130 min. Operating at these conditions results in an overall coal conversion approximately equal to 92%, oil and asphaltene yields ranging from 40 to 45%, and a 90% conversion of the initial sulphur into H₂S.     

Carbon-air electrode with regenerative short time overload capacity: Part 1. Effect of manganese dioxide

To increase the overload capacity of the carbon-air electrode in alkaline electrolyte, without seriously changing the electrode potential during short-time overload periods, small amounts of MnO₂ were added to carbon and the effects investigated. The potentiodynamic investigations revealed the existence of two reducible manganese species with reduction potentials of about –280 mV and –400 mV. The second potential corresponds to the known reduction process Mn(III) Mn(II) whilst the first one corresponds to a reversible process (the product is easily reoxidized by air or anodically), not reported in literature as yet. It was explained in terms of a new redox potential for the system Mn(III)/Mn(II), both species being strongly and irreversibly adsorbed at the carbon surface. The galvanostatic discharge curves in the absence of air indicate that a sufficient overload capacity of about 40 C cm^–2 is available if 22% (wt) MnO₂ is added. Steady-state polarization curves in the presence of air indicate the catalytic effect of small amounts of MnO₂ for the oxygen reduction reaction.     

Deuterium as a tracer in coal liquefaction Part 2. Non-catalytic studies

A bituminous Australian coal (Liddell) was liquefied in the absence of catalyst using tetralin as vehicle, and molecular deuterium and hydrogen—deuterium gas mixtures. The structures of the liquid and gaseous products were investigated by mass spectroscopy, ¹H-and ²H-NMR spectroscopy and gel permeation chromatography (GPC). The proportion of ²H to ¹H in the liquid products was found to be higher at 425°C than at 400°C because deuterium preferentially enters more aromatic rings at the higher temperature.The distributions of deuterium in the deuteromethanes formed during liquefaction show that deuterium randomly enters the structural groups in the coal which produce methane before the methane is released to the gas phase. This illustrates the extreme mobility of hydrogen, including the hydrogen that originates from the coal. As a consequence, it is proposed that hydrogen released as methane arises from a pool in which memory of the original bonding is lost.     

Distribution of zinc chloride catalyst in products from short residence time liquefaction of clear creek,Utah coal

The total recovery of zinc chloride decreases from 94.4 wt% to 75.4 wt%, on the basis of ZnCl₂ catalyst added, with the progress of the liquefaction reaction. Over 90 wt% of zinc and chlorine found in the liquid and solid products is present in the toluene insolubles. The contents of zinc and chlorine in the other products are below 1 wt% and 5 wt%, respectively, of the ZnCl₂ added.     

Kinetics and reaction scheme of coal liquefaction over molten tin catalyst

Rates of formation of gases, oils, asphaltenes and preasphaltenes during non-solvent liquefaction of coal over molten tin catalyst have been measured. A probable reaction scheme and the rate constants for the pathways comprising the scheme have been presented. The results show that the catalyst greatly accelerates the conversion of preasphaltenes to asphaltenes. It also accelerates two other reactions, i.e., coal to preasphaltenes and coal to asphaltenes. By contrast, the catalyst does little to promote gasification and formation of oils.     

Filtration in coal liquefaction: 1. Influence of coal type

Commercial solvent extraction systems for coal must be able to process coals with various properties. In this study the influence of coal type upon the extraction yield of coal and the filtration of extraction products has been investigated. All the coals used gave high extraction yields in hydrogenated solvent but resultant products exhibited considerable differences in rates of filtration. This variation in filtration rates is dependent upon particle size, concentration and composition of the residual solids. No simple relation was found to relate coal type with filtration rate.