Coal product, solvent fractionation

Figure 1. Flow diagram for solvent fractionation of coal hydrogenation product...

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. 

The research involved the use of deuterium gas as a tracer to follow the incorporation of hydrogen into coal. Neither donor solvent nor catalyst was used in those experiments. The liquefaction product was solvent fractionated, and the fractions were examined for deuterium incorporation in each structural position. 

Stirring was initiated, and the autoclave was heated to 400°C which required 90 minutes for E10 and 100 minutes for El9. The temperature was maintained at 400°C for 1 hour, then lowered to room temperature. The cooling duration to 300°C was 5 minutes for E10 and 40 minutes for El9. Stirring was terminated at room temperature. Gaseous products were removed for analysis by gas chromatography coupled with mass spectrometry (GC-MS). The reaction products were distilled at reduced pressure to remove the spent donor solvent mixture, and the remaining coal products were solvent fractionated. 

The atom % 2H values of the solvent-fractionated products are also shown in Table I. In previous hydrogenation experiments conducted without the use of a donor solvent (1,18), deuterium incorporation increased from the most soluble oil fraction to the insoluble residue. In E10 and El9, contact of the coal with Tetralin resulted in a uniform incorporation of deuterium in almost all of the four product fractions. In El9, the BMI fraction s high value of 61 atom % 2H may be due to direct gas-phase exchange and deuteration. 

Table V summarises the data of the sulphur analysis of the hydrocracked liquids and the various bpt fractions for CoMo and NiMo catalysed experiments. The sulphur contents of neither the total hydrocracked liquids nor the individual bpt fractions showed any dependence on repeat contact or catalyst type. The values did show that the sulphur concentrated in the recycle solvent fraction (275-450°C), suggesting that, even under the relatively strong conditions used, certain sulphur-containing compounds will survive to be recycled in the solvent However, the sulphur content of the coal liquid feed was reduced by about 50% and the sulphur content of the likely upgradable product was low.

Asphaltene is an essential component of any dark-colored, heavy, viscous and nonvolatile oil, regardless of the oil source. Asphaltene can be obtained from the oil extracted from a naturally occurring organic-rich fossil material by a simple solvent fractionation. Asphaltene also can be obtained from the chemical conversion product of a solid fuel, such as pyrolysis or catalytic hydrogenation of coal or shale. The former is an example of the asphaltene isolated from native petroleum oil. An example of the latter is the asphaltene obtained from a synthetic crude, such as shale oil or coal liquid. 

The solvent fractionation scheme for separating coal liquid products into flve fractions propane-soluble (oil) propane-insoluble and pentane-soluble (resin) pentane-insoluble and benzene-soluble (asphaltene) benzene-insoluble and carbon disulfide-soluble (carbene) and carbon disulfide-insoluble (carboid) was described previously (1,2). 

In our plan of attack we first depolymerized the coal by using conditions suggested by Imuta and Ouchi (11) then we divided the product into five fractions of progressively increasing polarity, using solvent fractionation. These fractions then were analyzed separately by... 

A portion of the coal product (168 mg) was dissolved in pure tetrahydrofuran (2 mL) and chromatographed on Styragel GPC columns (Waters Associates). Columns with a molecular weight exclusion limit of 10,000 (2 X 61 cm) and 2,000 (2 X 61 cm) were connected in series. Tetrahydrofuran was used as the mobile phase (0.36 0.01 mL/min). About 30 fractions (3.7 to 3.8 mL) were collected in each experiment. The tetrahydrofuran was removed in vacuo, and a stream of filtered, dry nitrogen was used to remove the final traces of the solvent. The coal product obtained with labeled butyl iodide was partitioned into 17 fractions (total weight, 178 mg). Samples to be used for NMR spectroscopy were dried thoroughly at 25°C at about 5 torr for 40-45 hr to remove the remaining traces of tetrahydrofuran. 

UV and fluorescence detectors can be used for SFC. Carbon dioxide and nitrous oxide have made possible the use of conventional GC flame detectors (24). Based on studies of aliphatic fraction of a solvent refined coal product with supercritical CO2 at 40 C and conventional FID detection, it was concluded... 

From a more practical viewpoint, the solvent extraction of bituminous coals has been used as a means of coproducing clean liquid transportation fuels as well as solid fuels for gasification. Coal solvents are created by hydrogenating coal tar distillate fractions to the level of a fraction of a percent, thus enabling bituminous coal to enter the liquid phase under conditions of high temperature (above 400°C [750°F]). The pressure is controlled by the vapor pressure of the solvent and the cracked coal. Once liqnefied, mineral matter can be removed via centrifnga-tion, and the resultant heavy oil prodnct can be processed to make pitches, cokes, as well as lighter products. 

The overhead liquid product is fractionated into light and heavy liquids and the residue is de-ashed by treatment with a solvent that promotes the settling of an under-flow stream that contains, essentially, all of the ash that occurred in the original coal feed. 

Figure 17.28 shows a schematic of the SRC-I process. The feed coal is crushed and mixed with a recycle solvent and hydrogen prior to entering the preheater. The preheater effluent, at 700 to 750°F (370-400°C), then is fed to the dissolver unit, or thermal liquefaction unit (TLU), which operates at 840 to 870°F (450-465°C). There is no upgrading step, as the desired product is a solid at room temperature and not a distillate. The solids removal from the liquid slurry is accomplished by critical solvent de-ashing (CSD). The solids-free resid from the CSD was separated by vacuum distillation into a recycle solvent (the light fraction) and a solvent refined coal product (the bottoms). 

A flow diagram of the solvent-refined coal or SRC process is shown ia Figure 12. Coal is pulverized and mixed with a solvent to form a slurry containing 25—35 wt % coal. The slurry is pressurized to ca 7 MPa (1000 psig), mixed with hydrogen, and heated to ca 425°C. The solution reactions are completed ia ca 20 min and the reaction product flashed to separate gases. The Hquid is filtered to remove the mineral residue (ash and undissolved coal) and fractionated to recover the solvent, which is recycled. 

The product workup consisted of continuously extracting the filter cake with tetrahydrofuran (THF) and combining the THF and filtrate to make up a sample for distillation. In some experiments the THF extracted filter cake was extracted with pyridine and the pyridine extract was included in the liquid products. Extraction with pyridine increased coal conversion to soluble products by an average of 1.6 weight percent. The hot filtrate-THF-pyridine extract was distilled. Distillation cuts were made to give the following fractions, THF (b.p. <100 C), light oil (b.p. 100-232 C), solvent (b.p. 232-482), and SRC (distillation residue, b.p. >482 C). 

Solvent Composition and Recovery. The solvent was defined as the product fraction which is soluble in the hot filtrate and the THF extraction of the filter cake and which boiled between 232 C and 482 C at atmospheric pressure. One of the requirements of a commercial coal liquefaction process is that a least as much solvent be created as is used in the process. In addition, the composition of the solvent must be kept constant if it is to be... 

The nmr analyses of the bottoms products given in Table IV show the material to have a large aliphatic content. The aromatic/aliphatic ratios of the fractions are higher than for the whole coal because of the presence of combined phenol reaction with Tetralin reduces these ratios considerably, presumably by transfer of much of this material to the solvent-range product, but some of it must remain in the bottoms as the aromatic/aliphatic ratio of the composite bottoms product from the fractions is higher than that from the whole coal. It was not possible to calculate the contribution that the diluents, excess solvent and combined phenol, made to the aromatic H, but the large monoaromatic content of the bottoms product must be due, in part, to these. 

The solvent-range product was not separately analysed as it was not able to be separated from the recovered solvent in the distillate. However, GLC examination of the distillate indicated that the solvent-range product was derived mainly from aliphatic side chains in the coal (9). Note that virtually no solvent-range product was derived from fraction D. 

However, as pointed out above, the commonly proposed free radical mechanism is not entirely consistent with the observed behavior of H-donor solvents and coal. Further, a thermally promoted C-C or C-0 bond-scission is inconsistent with our observations in the -PrOH work at 335°C. As also mentioned, a major fraction of the coal was converted in this system to a product with a number-average molecular weight of less than 500. If we consider that the rate constant for the unimolecular scission of the central bond in bibenzyl is expressed (5) as... 

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