Retort Fischer assay

Sample preparation for the modified Fischer assay technique, a standard method to determine the Hquid yields from pyrolysis of oil shale, is necessary to achieve reproducible results. A 100-g sample of >230 fim (65 mesh) of oil shale is heated in a Fischer assay retort through a prescribed temperature range, eg, ca 25.5—500°C, for 50 min and then soaked for 20 min. The organic Hquid which is collected is the Fischer assay yield (7). The Fischer assay is not an absolute method, but a quaHtative assessment of the oil that may be produced from a given sample of oil shale (8). Retorting yields of greater than 100% of Fischer assay are possible. 

A Fischer assay simulates the conversion of oil shale to usable fuels in an above-ground retort. The results of an extensive program of chemical analysis of major and trace elements in spent shale, oil, and water collected from the Fischer assay of a standard oil shale are presented. The concentration of major elements in raw and spent shale can be determined only to 10% in this study. Two criteria show that fluorine and zinc may have been mobilized during the assays. The concentrations of arsenic and selenium in the Fischer assay retort water exceed the maximum permissible concentrations for drinking water. 

CH4, and H2S contents with less C3-C6 and CO2 contents. The lower C3-C6 levels in the case of the eastern oil shales are due to a more efficient condensation of those gases to the liquid product in our assay procedures (12-13). Gas cleanup would ultimately be expected to yield a higher Btu gas for the eastern oil shale-derived gas than a corresponding western gas due to elimination of H2S in the former and high CO2 levels and condensation of much of the C4-C6 range in the latter. Finally, it should be emphasized that the compositional information presented is based on Fischer assay retorting procedures and may not be entirely representative of products produced under commercial scenarios. 

Figure 2. Particle-size effect on hydrocarbon yields at 930° F fluidized-bed retort (%) Fischer assay retort (----------------------------------).

Value assumed from Fischer assay and moisture content. The addition of steam to the process prevented accurate measurement of water produced in retorting. 

A total material balance assay is a Fischer assay in which the retort gases are collected. A complete material balance closure and yields in excess of those expected from Fischer assay results are achieved. More complete descriptions of both the Fischer assay and the Tosco material balance assay methods have been reported (9). 

Shale oil and a fuel gas have been produced by microwaveheating oil shale in a standard microwave oven in conjunction with experimentation to develop an in situ microwave retorting process. Various grades of oil shale have been subjected to high microwave fields. The derived oil has been submitted to various physical and chemical testing methods, and the chemical composition of the evolved gas has been evaluated. The specific gravity pour point yields of oil, water, gas, and losses and spent shale are compared with parallel data obtained with the Fischer assay procedure. Important differences in oil flow properties and gas composition are discussed in view of microwave interactive theory. 

It has been noted that shale oil properties can vary with the percentage yield of a thermal process. The percentage of oil recovery compared with the Fischer assay is presented in Table III, which shows some more characteristics of the retorting runs of Table II. The data... 

Low Ethylene Ethane Ratio. Examination of the C2 fraction indicated an ethylene ethane ratio of 0.5 for the total gas, projected retorting indicies would suggest a longer residence time, or an effective temperature slightly higher than Fischer assay (18). 

The oil yield results of the Fischer assay obtained by the modified TOSCO procedure have been discussed previously (16) and are summarized in Table I. The relative standard deviation in this set of assays was 2.5% on nine oil-yield assays. TOSCO has obtained precision limits of 0.6%. Further examination of the weight fractions in 46 retort runs showed that the major source of this scatter was in the weight of liquid product obtained. This probably can be attributed to the inadequate control of the retort temperature program. The above summary indicates... 

Retort water collection and preservation poses special problems because of its unusual chemical characteristics. For example, the electrochemical properties of the Fischer assay water collected in this study were pH = 8.95, conductivity = 29,000 fjmho/cm at 25°C, and Eh = —310 mv. The major species in the water are NH4 and HCOa". Con-... 

The Fischer assay oil was free of particles. Little residue was found upon filtering a 500-mL sample and the differences in elemental analyses between the filtered and unfiltered oil were within analytical error. However, this may not be the case for oil from other retorts and filtration may be necessary. Nitrogen gas was bubbled through the samples and they were stored in Tefion containers in a refrigerator. 

The analytical results for trace elements appear better than those for the major elements. Table IV is a summary of results for the raw shale (OS-1), and the spent shale from the TOSCO II pilot plant (SS-2), and the spent shale (FS), unacidified water and unfiltered oil from the Fischer assay studies. The spent shale SS-2 is the residue from the retorting of the same raw feedstock from which OS-1 was taken. In the case of the solid samples, the concentrations are from multiple analyses of different splits. In all cases, the relative deviations of multiple analyses lie within 10%. Comparison of these results with those from other laboratories on different splits of the same solid samples show agreement within 2cr SI), The two exceptions are manganese and zinc for which the results reported here are low in comparison with other methods. Analyses of NBS standard coal (SRM 1632) and coal fly ash (SRM 1633) are included also in Table IV. The concentrations in raw and spent shale are similar to those reported by TOSCO except for selenium where their values range from 10-16 ppm (3). 

In some instances, the RI value obtained is a large negative number. This is usually an indication that the retort process has added that respective element or that contamination occurred during sample handling. In this study, care was taken to minimize this problem. All the retorts and aluminum inserts used in this Fischer assay were scrubbed and rinsed with HNO3 (15). All new glassware was used and was also rinsed with HNO3 prior to use. All equipment was used in a Fischer assay of OS-1 before samples were collected for the mass balance studies. 

Mass Balance Considerations. The values of ER for the Fischer assay spent shale are contained in Table V. If it is assumed that the relative standard deviation in the analyses is 10%, then the relative probable error in ER would be 14% if the analytical errors were indeterminant and 20% if the errors were determinant (38). The mass ratio of OS-l/FS is 1.24 as derived from the assay data in Table I. It is not possible to conclude that any trace elements are mobilized from the solid material during the assay retorting. The ER results obtained for arsenic, selenium, and molybdenum indicate the importance of analytical precision in detecting any trace element mobilization during oil shale retorting. The values of RI contained in Table V show a similar dependence on analytical precision. The probable errors in these values are also between 14 and 20% if the relative standard deviation in the analytical results is assumed to be 10%. These results indicate that, within experimental error, none of the trace elements have been lost during Fischer assay. More definitive conclusions on whether elements are mobilized or lost can only be reached with more precise analytical... 

The modified Fischer Assay (USBM RI 6676 and more recently ASTM D.4904) has been used for determining grade of oil shale. From experience with the Queensland shales, an 80 g charge is used in the Fischer retort. The half core 2 metre interval provides from 3 to 5 kg of sample and the assay rejects are retained and provide a sample bank for characterisation tests carried out on the oil shale seams. 

Oil shale (33 gal/ton by Fischer assay) from the Piceance Rasin (Green River Formation, Colony mine) in Colorado was crushed and sieved to -16/+60 mesh. Retorting was carried out in a fixed bed reactor at a pressure of 2.6 MPa (380 psig) with a slow (6°C/min.) heat up to 600°C followed by 10 min. at 600°C. Short gas residence times were used to minimize secondary oil degradation reactions. Gases were analyzed by GC using a Carle 157A refinery gas analyzer. Liquids were collected in a cyclone, and the water was separated from the oil by distillation. 

The Fischer assay is a standardized laboratory test in which an oil shale sample is retorted at 500°C to determine its oil yield. It provides a measure of the grade of oil shale being processed. Commercial processes such as The Oil Shale Corporation s TOSCO II process give oil recoveries (synthetic crude or syncrude) of up to 100% of the Fischer assay. Oil and gas formation from... 

Figure 7. Effect of cracking on the ethene/ethane ratios of the total evolved gases, shown as a function of (a) cracking losses and (b) cracking temperature. The result indicated by A is for a Fischer assay. The other points indicate cracking over burnt shale (O), retorted shale (%), and in an empty reactor ([2). The ratios correlate better with the cracking temperature than with the cracking losses.

The LLL kinetic model predicts that the maximum achievable oil yield is that of Fischer Assay and that the coke associated with Fischer Assay is stoichiometrically related to the kero-gen. This assumption may be appropriate for in-situ retorting, but it is not applicable to the present conditions where oil yields higher than Fischer Assay are measured. Oil yields as high as 110% of Fischer Assay have been reported for high heating rates (Hinds, j)) Therefore, another objective of this work is to extend the kinetics of oil production beyond the Fischer Assay limit. 

A practical implication of the results of this work is that Fischer Assay yield is probably a reasonable upper limit for any retorting process. This work has shown that a very small particle size increases oil yield and decreases coke yield, but long reaction times are necessary. Also, low coke yields may not be desirable from overall heat balance considerations if the coke is to be used as an energy source for the process. Lowering the temperature also increases oil yield but at the expense of the gas yield and with the requirement of long reaction times. 

Because the Fischer assay does not provide information about what chemical properties of oil shales are important for producing liquids, a number of attempts have been made to correlate various chemical and physical property measurements with oil yields determined by the Fischer assay. The assay procedure has been modified for different types of oil shale, or to more closely resemble a certain type of retorting process. ... 

The grade of oil shale can be determined by measuring the yield of oil of a shale sample in a laboratory retort. This is perhaps the most common type of analysis that is currently used to evaluate an oil shale resource. The method commonly used in the United States is the modified Fischer assay, first developed in Germany and then adapted by the U.S. Bureau of Mines for analyzing oil shale of the Green River formation in the western United States (Stanfield and Frost, 1949). The technique was subsequently standardized as the American Society for Testing and Materials (ASTM) Method D-3904-80 (ASTM, 1984). Some laboratories have further modified the Fischer assay method to better evaluate different types of oil shale and different methods of oil-shale processing. 

Dyni, J.R., Anders, D.E., and Rex, Jr., R.C. (1990). Comparison of hydro-retorting, Fischer assay, and Rock-Eval analyses of some world oil shales, in Proceedings 1989 Eastern Oil Shale Symposium. University of Kentucky Institute of Mining and Minerals Research, Lexington, pp. 270-286. 

Stanfield, K.E. and Frost, I.C. (1949). Method of Assaying Oil Shale by a Modified Fischer Retort, Report of Investigations 4477. U.S. Bureau of Mines, Washington, DC. 

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