Chapter 17 Hydroliquefaction of Coal Liquids in Spinning and Falling Basket Autoclaves
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Graham Harrison, P. W. Doughty, and S. S. Ali Department of Applied Sciences, Staffordshire Polytechnic, Stoke-on-Trent ST4 2DE, United Kingdom Fresh and used catalysts (CoMo, NiMo, ZnMo and ZnW) were used in hydroliquefaction experiments with batches of Point of Ayr coal liquid. One series of experiments was carried out at 400°C for 2 h with all four of the catalysts using five repeat contacts. A second series used only CoMo over three repeat contacts and considered variations in reaction time and temperature. The performance of the two spinning/falling baskets was compared by investigating the hydrogenation/hydrocracking of phenanthrene at different stirrer speeds, reaction times and reaction temperatures. The analysis schedule made use of vacuum distillation, gas chromatography, carbon, hydrogen and sulphur analysis and specific surface area analysis. Conversions were assessed from the vacuum distillation and gas chromatography data, making use of marker compounds.
Aspects of coal liquefaction have been much researched, particularly with there-emergenceof interest caused by die oil crisis in the 1970's. The type of reactors used in the studies has been various, ranging from small 'bomb' type microautoclaves through larger autoclaves and benchscale reactors to larger scale pilot or demonstration plants. The use of differently sized and designed high pressure equipment for liquefaction studies further complicates an already complex system and allows only limited comparison of results. In a continuous reactor, particularly of the trickle bed type, intimate contact between the coal liquid and the catalyst will be maintained throughout the pass of the liquid feed. In an autoclave, particularly of the stirred design, the contact between the liquid and the catalyst will not be as intimate. The action of the stirrer will produce a centrifugal force which will tend to throw the liquid away from the catalyst surface. Consequemly, it can be visualised that less strongly adsorbed molecules will spend a shorter time at the catalyst surface so that reaction rates and mechanisms could be very different from those observed in continuous reactor studies. In addition, steady state conditions can be readily investigated in a continuous reactor, whereas for a single contact in an autoclave, steady state conditions may not have been established and changes in catalyst activity will become more relevant. Autoclaves provide reactors which can be used readily to acquire data from coal liquefaction studies but are less representative of likely commercial plant type reactors than small scale continuous bed-type reactors. Ideally comparisons between reactors are best made by carrying out experiments in various designs ofreactorsunder similar reaction conditions, but, in order to cover the full range of designs adequately, a larger expenditure on equipment (beyond the budgets of most laboratories) would be necessary. However, steps can be taken to cover the
0097-6156/91/0461-0225$06.00/ϋ © 1991 American Chemical Society
Schobert et al.; Coal Science II ACS Symposium Series; American Chemical Society: Washington, DC, 1991.
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problem by finding whether single contacts in a stirred autoclave can be representative of steady state conditions after running for a particular time or how many contacts at a particular time are required to achieve a steady state condition, i.e. constant catalyst activity. Extrapolating results from autoclave studies to continuous reactors should then become more realistic.
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EQUIPMENT FOR E X P E R I M E N T A L P R O f i R A M M E A l l the hydrocracking/hydrogenation experiments were carried out in 500 ml capacity spinning type autoclaves. Two autoclaves of this design were used and the autoclaves were compared in experiments using the model compound phenanthrene, chosen because phenanthrene and its hydro-derivatives represent a large proportion of the solvent which is recycled in coal liquefaction processes. The design of the autoclaves is relatively unique as the extrudate catalyst is retained in a squat wire mesh basket. The basket is kept above the liquid phase by a bar which can be retracted manually when the basket falls down the magnetically driven stirrer shaft into the liquid phase. The cross-section of the stirrer shaft is such to allow its rotation without moving the basket in its upper position but, on the basket entering the liquid phase, the cross-section changes so that the basket rotates together with the shaft The stirrer speed is variable and is controlled at a separate box, to which the autoclave control and measuring thermocouples are also attached. The two thermocouples seat alongside each other in a single well and one is used as an override protect; the override temperature is generally set 60-70°C above the control temperature. The autoclave is heated by a quick-release external unit which has three individually wired elements. Control of the heaters is again effected from the separate control box and the heaters are capable of increasing the temperature from ambient to the working temperature (generally in the range 400-450°Q in about 45 mins. At working temperature, only two of the heaters are required to maintain the temperature within a range of about 20°C. A n exception to this control range occurs on initial release of the basket when an overshoot of 40-70°C can occur. To minimise this overshoot, the basket is released about 50°C below the reaction temperature. The overshoot is probably caused by initial exothermic cleavage of bonds linking aromatic centres and is different for the two autoclaves. This point is considered further later in the chapter. To be representative of actual process situations, the hydrocracked product was primarily separated by vacuum distillation into acetone/dry ice cold trap fractions and >275,275-300 and 300-475°C fractions. (Occasionally the top split was taken at 450°C.) Samples of the various fractions were injected through an *on column' pneumatically sealed assembly onto a 25 m, 0.32 mm 0 V 101 chromatography column fitted to a Perkin-Elmer Sigma 3B Chromatograph. The temperature was programmed at a ramp rate of 5°C/min between the initial and final temperatures. Quantitative values of yields were calculated by making use of markers to separate the hydrocracked product into 13 groups as indicated in Table I. Selection of these groups was made from the compounds available and some group titles must be viewed as nominal only, whereas other group definitions are more precise. Apart from group 3 (naphthalene), group 9 (phenanthrene), group 10 (anthracene) and group 11 (pyrene), all groups contained many compounds. Probably the groups with the largest uncertainty were groups 11 (hydropyrenes) and 13 (material boiling above pyrene to the top cut temperature). Two product ranges, defined as material bpt 218-260 >260-270 >270-280 >280-295 >295 - 315 336 340 >340-400 400 >400 - 475/450
>220-380 >380-510 >510,580, 680 >580 - 680 >680-910 910 950 >950 - 1280 1280 >1280
The distillation fractions were also analysed for their carbon and hydrogen contents using a Leco C H N Determinator which was also used for similar analysis of the used catalysts. The hydrocracked liquid and the used catalysts were analysed for their sulphur contents using a Leco Sulphur Determinator. Some specific surface area analysis by nitrogen adsorption was carried out on the used catalysts using a Micromeritics instrument REPEAT CONTACT EXPERIMENTS AT CONSTANT REACTION PARAMETERS EXPERIMENTAL PROCEDURE In this series of experiments, the catalysts were used over five repeat contacts with fresh coal liquid. Point of Ayr coal liquid was supplied by the British Coal Corporation, Coal Research Establishment (CRE); one batch of this coal liquid was used in experiments with CoMo and N i M o catalysts and a further batch was used in experiments with ZnMo and ZnW. The catalysts were prepared as extradâtes by the technique of incipient wetness which requires stirring the dry alumina support with a set volume of a pre-determined concentration of an appropriate soluble salt of the metal such that the pore space is just taken up by the metals at the required concentration. The alumina support was supplied by Akzo Chemie, The Netherlands and the catalysts were made up to contain 15% WO3 or M0O3 and 3% NiO, CoO, or ZnO, expressed as a weight percentage of the weight of support For each of the experiments, 4.0 g of catalyst was placed into the basket which was then attached to the autoclave which had been loaded with - 100 g of the coal liquid. After sealing the autoclave, it was pressurised to 120 bar with hydrogen. The temperature was raised to the operating temperature of 400°C, at which it was maintained for a further two hours. The basket was released when the temperature reached 350°C and the stirrer speed was maintained at 650 rpm throughout the 2 h ran time. At the end of the experiment, the stirrer and heaters were switched off and the autoclave allowed to cool to room temperature. The autoclave was opened and the liquid contents were removed by suction. The basket was detached and the catalyst removed into a beaker which was placed in an ultrasonic bath. 10 c m of dichloroethane was added to the beaker which was agitated in the ultrasonic bath for 15 mins. The solvent was removed by suction filtration, the catalyst was washed with portions of acetone until the washings were colourless and the catalyst was air dried. The liquid product was transferred into a glass jar which was sealed under nitrogen and kept in a refrigerator until required. In order to have a sufficient amount of catalyst for a duplicate experiment on the five contacts and to allow a sample of catalyst to be 3
Schobert et al.; Coal Science II ACS Symposium Series; American Chemical Society: Washington, DC, 1991.
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retained after each contact for analysis, the following programme of 19 experiments was adopted:- 1st contact - 5 experiments; 2nd contact - 5 experiments; 3rd contact - 4 experiments; 4th contact - 3 experiments; and 5th contact - 2 experiments. Part of the hydrocracked liquid was vacuum distilled into the fractions defined previously. Initially, a distillation was carried out for each of the 19 experiments, but later a sample was taken from the product combined from each of the duplicate experiments making up the contact The various fractions were stored in screw cap vials in a refrigerator. For the G C analysis, a sample of each of the fractions was used as an approximately 1% solution (v/v) in cyclohexane. REPEAT CONTACT EXPERIMENTS AT CONSTANT REACTION P A R A M E T E R S RESULTS AND TRENDS A n example of the data, broken down into the 13 product groups, calculated from the distillation and G C analysis of the hydrocracked liquids from CoMo-catalysed experiments is shown in Table II. It can be seen that the distribution of each of the contacts is similar, reflecting no dependence onrepeatcontact even in the case of the first contact which used fresh catalyst. This situation was generally observed for the other catalysts used and a summary of the results for the four catalysts is shown in Table III. The results for ZnW could be interpreted as a gradual decrease in conversion to material bpt
