Fischer assay yield

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. 

TABLE 27-3 Fischer-Assay Yields from Various Ranks of Coal (As-Received Basis)... 

Deposit Unit Location Fischer Assay Yield (LTOW)... 

Illustrative Fischer assay yields and associated oil and gas composition data are presented in Table 2 for both eastern (10) and western (8) oil shales of nominally similar organic carbon content. The eastern oil shales yield more gas and water and less oil than the western oil shale used for comparison. The major differences in the oil composition include comparatively lower nitrogen and higher sulfur contents for the Sunbury and Cleveland shale oils. The lower H/C ratio for the eastern oils indicate a more aromatic oil, as reported previously (11). Assay gas composition for the eastern oil shales indicate higher H2,... 

Figure 5. Plot of Fischer assay yields and carbon content for D series oil shales. Regression data are Sunbury Shale, oil yield - 1.03 (%C) - 0.69 (r =. 97) and Cleveland Shale, oil yield- 1.18 (%C) - 1.16 (r =. 97).

TABLE II. FISCHER ASSAY YIELDS WITH OIL AND GAS COMPOSITION DATA FOR EASTERN VS. WESTERN OIL SHALES... 

Experiments were conducted to determine the effect of oil shale particle size on product yields at 930°F. The yields obtained for particles of six different sizes are compared with Fischer Assay yields in Figure 2. It is apparent that oil yields higher than Fischer Assay are obtained for small particles whereas large particles produce Fischer Assay yield. The incremental oil produced from small particles is balanced by a decreased coke make while the gas make remains constant. The oil yield appears to have a limit at about 110 wt % Fischer Assay, but this may be entirely due to the limited range of particle sizes investigated. It is possible that the oil yield would increase further for, say, 10 tm or 1 im particles. However, particles of this size could not be studied in the apparatus of this work. 

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. 

Table 10. Average Yields and Range of Yields of Fischer Assay of Various Coals ...

Properties. A high volatile western Kentucky bituminous coal, the tar yield of which by Fischer assay was ca 16%, gave a tar yield of ca 26% at a pyrolysis temperature of 537°C (146—148). Tar yield peaked at ca 35% at 577°C and dropped off to 22% at 617°C. The char heating value is essentially equal to that of the starting coal, and the tar has a lower hydrogen content than other pyrolysis tars. The product char is not suitable for direct combustion because of its 2.6% sulfur content. 

Properties. Results for the operation using subbituminous coal from the Wyodad mine near Gillette, Wyoming, are shown in Table 13. Char yields decreased with increasing temperature, and oil yields increased. The Fischer assay laboratory method closely approximated the yields and product assays that were obtained with the TOSCOAL process. 

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). 

The Fischer assay is an arbitrary but precise analytical tool for determining the yield of produces from low-temperature carbonization. A known weight of coal is heated at a controlled rate in the absence of air to 773 K (932°F), and the produces are collecled and weighed. Table 27-3 gives the approximate yields of products for various ranks of coal. 

All of the tests conducted in the pilot plant were with a single batch of Colorado oil shale from the U.S. Bureau of Mines mine at Rifle, Colo. Mine run material was crushed to 1/4-in. size at the mine and sieved elsewhere into various fractions. We selected a —6+10 U.S. Standard sieve size material for most of these tests because it was the largest size which we could successfully feed with our existing equipment. This material had a Fischer assay oil yield in the 20-25 gal/ton range. 

Procedure. The site was developed on a five-spot pattern about 25 ft square, as shown in Figure 2. The wells were rotary drilled with water and completed with 50 ft of 7-in. casing and cemented to the surface. A 6%-in. hole was drilled below the casing to a total depth of 100 ft in the oil shale. Two additional wells were drilled off pattern as observation wells. A Fischer assay determined the oil yield of the section as 19.0-26.5 gal/ton. Two sand-propped hydraulic fracture treatments were applied for emplacing NG1 in the formation. 

Procedure. Green River site 1 was located 5 mi west of the Rock Springs sites 4 and 5. The oil shale zone of interest, at approximately 346-385 ft, was selected after studying the analysis of cores cut from an earlier well. As determined by Fischer assay, oil yield of the cored section averaged about 21.0 gal/ton. 

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... 

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... 

Nothing has been said of the spent shale SS-2 which was produced from the TOSCO II pilot plant. This is because it is diflBcult to say how representative the 50 lb of OS-1 is of the 100 tons of —1/2 in. mesh feedstock. TOSCO determined that the 100 tons assayed at 37-38 gal/ton of oil. The Fischer assays performed in this study yield a value of 39.8 gal/ton (16). The difference in oil yields is outside of analytical uncertainties (16). If OS-1 can be considered representative of the pilot plant feedstock, then the abundances of trace elements in SS-2 compared with those in OS-1 would give ER values greater than unity for all trace elements. The elements fiuorine and zinc in SS-2 have ER values of 1.05 and 1.17, respectively. Based on these results, it appears that the trace elements for which data are reported here are not mobilized during the TOSCO II process. 

The Kettle Point Formation subcrops in southwestern Ontario. It is up to 60 m thick, with organic carbon values usually in the range 5%-16% and Fischer Assay oil yields up to 70 1/t (17 gal/ton). The organic matter is dominantly kerogen of marine origin and has attained only an immature thermal maturation stage. A particularly rich upper zone is present over much of the subcrop area and its thickness seems to be controlled by post-Kettle Point erosion. Although studies are still preliminary, especially for the Devonian shales, the Kettle Point Formation appears to have the most potential for shale oil production in southern Ontario. 

The economic potential of these black shales is being assessed partially by measurement of the total hydrocarbon yield upon pyrolysis (YP) and by Fischer Assay (FA). These parameters are then being related to TOC content, so that the economic potential can be assessed on a regional scale from the abundant TOC data being gathered from borehole cores. 

The relationships between TOC (%C) and Fischer Assay oil yield (1/t) are shown in Figure 2 and the correlation equations are ... 

The pyrolysis method (YP) was developed as a convenient, rapid alternative to the Fischer Assay. Major disadvantages include the small sample size (5-20 mg) which may not be representative, the poor control of absolute temperature, and the fact that all hydrocarbons—gases and liquids at room temperature —are detected. Thus, the YP provides a relative yield of hydrocarbons which cannot be directly translated into FA oil yield. 

Figure 2. The relationship between Fischer Assay oil yield and total organic carbon (TOC) content for shale samples from the Whitby (left) and Kettle Point (right) Formations.

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