Monitoring oil shale retorts by off-gas alkenealkane ratios

Monitoring oil shale retorts by off-gas alkene/alkane ratios John H. Raley Oil Shale Project, Earth Sciences Division, Lawrence Livermore Laboratories, P.O. Box 808, Livermore, California 94550, USA (Received 16 March 1979)

Data from 125 kg and 6000 kg oil shale pilot retorts indicate that off-gas ethene/ethane and propene/ propane ratios are useful for monitoring retort performance. Information supplied by these ratios may be particularly valuable for in situ retorts. Since response time is short, the ratios may be useful for control as well as diagnosis of retort operation. The pilot retort data show that, for a smoothly operating combustion retort, the ratios provide an estimate of the effective shale heating rate. Rough operation, indicated by wide fluctuations in peak temperature, is reflected by fluctuations in the ratios. Particularly high ratios indicate exposure of oil to excessive temperature and/or oxygen with resultant loss of oil yield. A relation between elevated ratios and oil loss via conversion to gaseous hydrocarbons is demonstrated for retorting of shale blocks immersed in a bed of smaller shale particles. Chemical interpretation of the data is made by comparison with the thermal cracking of straight-chain hexadecane and pyrolysis of polyethylene. A free radical, chain reaction scheme is proposed. The dependence of alkene/alkane values on heating rate is attributed to competition between carboncarbon bond cleavage versus hydrogen atom-transfer processes. The contribution of oxidation reactions to elevated alkene/alkane values is also discussed.

Development and commercialization of in situ oil shale retorting would be aided by improved methods for monitoring retort performance. Valuable information can be provided by emplaced sensors measuring such variables as temperature, pressure, or flow. However, the resultant data are limited by the size of field which each sensor can cover and by economic constraints on the number of sensors implanted. Properties of retort effluents, however, may give lesser insight into local conditions but can also reflect integrated retort performance. Furthermore, the properties of gaseous effluents reflect retort conditions existing only shortly before the time of property measurement. Thus, they may be useful for retort control. For these reasons, the capability to characterize retort performance by measurements on the retort off-gas is a continuing objective of retorting studies. One example of the utility of off-gas composition data is the correlation between hydrogen content and the extent of char-H,0 and CO-H,0 reactions. The increased H, content of the off-gas when steam is included in the retort gas feed, especially with small shale particles, has been noted earlier.’ Also, the continuous increase in H, content as retorting progressed through the shale bed has signalled the growing importance of these reactions.’ RETORT INDEX Jacobson, Decora, and Cook’ proposed the use of ethene/ethane weight ratios in retort off-gas as a Retorting Index (RI) calculated from: 0016-2361/80/060419-06S2.00 @ 1980 IPC Business

Press

RI =

T(“F) =

1000 0.8868 - 0.4007 log (ethene/ethane)

The laboratory data used to develop this relation were obtained at high temperatures (575 and 703°C) high heating rates and in the presence of various sweep gases. Ethene/ethane weight ratios ranging from 0.7 to 3.7 were reported. The authors also compared calculated RI values with measured temperatures for retorting of small shale particles entrained in steam or steam-air mixtures. The substantially higher RI values, as compared to furnace temperatures, were attributed to pyrolysis of retorting products during their lengthy residence time after liberation from the shale. In applying the RI concept to combustion retorting in 6 and 150 ton/day units, the authors state that RI gives a more meaningful value for the temperature of retorting than does the much higher maximum retort temperature. They also reported that the RI follows oil yield more closely but gave no yield data. In their 10 ton combustion retort data, the RI falls both below and above maximum bed temperature. In their results RI (hence, ethene/ethane) is not a measure of retorting temperature only, but varies with heating rate, residence time and possibly copresence of oxygen. We have examined data from our 125 kg and 6000 kg shale combustion retorts for relations between alkene/alkane ratios and retort conditions. As described previously ‘y3 the shale held in these cylindrical vessels is in contact with an air-containing gas under downflow, simulated in situ (minimal heat loss) conditions. The heat required for releasing oil from the

FUEL,

1980,

Vol 59, June

419

Monitoring

oil shale retorts

by off-gas

alkenelalkane

ratios:

with time and vary with radial position at constant bed depth. Considering these uncertainties, it appears that using the off-gas alkene/alkane ratios and the curves in Figure 1 will give better estimates of the effective heating rates. In any event, the two procedures give heating rates in the 2-8°C min- ’ range which agree within a factor of about two. The constancy (or variation) of the off-gas alkene/alkane ratios during combustion retorting correlates with the smoothness (or roughness) of the retorting operation. Figures 2 and 3 compare the ratios with the peak gas temperatures as measured by a programmed, moving thermocouple that traverses the shale bed

shale is supplied principally by combustion of the carbonaceous solid remaining from oil release. CORRELATION AND HEATING

OF ALKENE/ALKANE RATE

J. H. Raley

RATIOS

125 kg Retort (0.3 m-diameter, 1.5 m length) In recent laboratory studies, Campbell, Koskinas, Gallegos and Gregg4 established a correlation between molar ethenelethane and propene/propane ratios and oil shale heating rate. Their data, obtained by heating powdered, 92 litre Mg-’ (22 gal ton - ‘) Anvil Points shale under self-generated atmospheres, are shown as curves in Figure 1. Table 1 lists off-gas alkene/alkane ratios, and the corresponding shale heating rates over the 350-500°C range, from four combustion retorting experiments in the 125 kg retort. The heating rates were estimated from thermocouples located along the axial centre line of the shale bed at each 10% of the 1.5 m bed depth. To minimize end effects, only data from the 20% through 80% depths were utilized. The mean values of the heating rates and alkene/alkane ratios are shown as points in Figure 1. In view of the different conditions for the two sets of data, agreement is reasonable. Obtaining direct measurements of the heating rates in the pilot.retorts is difficult. Especially with non-uniform size shale, the rates are non-rectilinear

-J500 Time

after

slartup

I h)

Figure 2 Off-gas alkenelalkane ratios and retort peak temperature during combustion retorting; expt. S-13 (Table I); shale particle size, -2.5 + 1.3 cm; gas feed, equimolar steam-air; 0, ethenelethane; 0, propenelpropane; n, peak temperature

‘.O

1’01’300

-11

0

0.2

01

. 0

I 1 Heoting

1 10 rate

I’C

min-’

) Time

Figure 7 Correlation of molar alkene/alkane ratios with shale heating rate; curves: powdered shale, autogenous atmosphere (Campbell, -; propenelpropane, - - - -; points: et al.‘), ethenelethane, offqas data, combustion retorting, Tab/e 1; 0, S-13; 0, S-14; n,S-15;n,S-16

Tab/e 7 Alkenelalkane ratios from 125 kg pilot retort experiments steam/air gas feed 0.7 m3 rnM2 minExperiment

s-13

Shale Size (cm) Shale Grade (litre Mg-1) Void Fraction Oil Yield (% of Fischer Assay) Mean Heating Rate (“C min-t) Mean Ethene/Ethane (mol) Mean Propene/Propane fmol)

420

FUEL,

1980,

-2.5+ 1.3 100 0.49 96 3.9 f 0.7 0.25 f 0.01 0.71 f 0.01

Vol 59. June

after

startup

Ih

I

Figure 3 Off-gas alkenelalkane ratios and retort peak temperature during combustion retorting; expt. S-15 (Table 1); shale particle size, -2.5 + 0.001 cm; gas feed, equimolar steam-air; 0, ethenel ethane; 0, propenelpropane; B, peak temperature

[Green

River Formation

shale from Anvil Points, Colorado]

S-16

S-l

5

-0.34 + 0.059 94 0.43 99 5.2 f 1 .O 0.27 f 0.01 0.73 f 0.02

-2.5 + 0.001 84 0.37 86 4.2+- 1.9 0.31 * 0.03 0.77 f 0.03

equimolar

s-14 -7.6 + 0.001 100 0.34 88 3.2+ 1.0 0.28 + 0.03 0.74 f 0.05

Monitoring

Table2 6000

kg retort experiment

oil shale retorts by off-gas alkenelalkane

L-l

Anvil Points Shale

Size (cm)

Grade (litre Mg-1)

Wt %

Crushed Blocks (equiv sphere, avg)

-7.6

100

61

120

39

+ 0.001

30

Void Fraction: 0.25 Gas Feed: Air + N2; 9 vol % 02; 1.2 m3 m-* Oil Yield: 72% of Fischer Assay

min-*

0.4 -

0.2 -

01.1

20

40 Time

after

60 startup

80

Ih

loo

I

Figure 4 Off-gas alkenelalkane ratios during expt. L-l (Table2); shale particle size, -7.6 + 0.001 cm and 30 cm blocks; gas feed, air + N2; 0, ethenelethane; 0, propenelpropane; , ethene/ethane $ range in S-l 5

behaviour, the alkene/alkane values in the off-gas reflected their presence. For in situ retorts, excessive ratio values may be a useful signal to initiate changes in operating parameters such as gas feed rate or oxygen concentration. In another experiment (L-2), exposure of blockproduced oil to excessive temperatures and/or oxygen was signalled by the off-gas alkene/alkane values. Six highly instrumented (17 thermocouples each), right circular, cylindrical blocks, of three different sizes, were immersed in matrices of smaller shale particles in the 6000 kg retort. The arrangement is illustrated in Figure 5. For all blocks, the bedding planes were perpendicular to the gas flow. The combined shale charge was retorted with an equimolar air-steam gas feed delivered to the top of the bed at about 0.7 m3 m-’ min- ‘. As shown in Figure 6, peaks in the ethenelethane ratio coincided with oil generation by the blocks (325-500°C; dashed lines). Furthermore, the peak areas scaled with block size. Propene/propane ratios behaved similarly. Because of the low thermal conductivity of oil shale, under normal heating rates the oil generated inside a block is exposed to higher temperatures as it moves to the block surface and escapes into the matrix environment. The size of these temperature increases is shown in Table 3 by measurements taken at a plane midway between the top (upstream) and bottom horizontal surfaces (Figure 5b). Especially with the largest blocks, the gradients and final temperature levels encountered by oil, generated within a block, suggest that appreciable oil loss could occur by thermal cracking. Figure 6 also provides evidence for oxidation of block-generated oil as it encounters the matrix en-

along the axial centre line.3 The nearly uniform-size shale (Figure 2, experiment S-13) produced much smoother plots of both peak temperature and alkene/alkane ratios than the wider size range material (Figure 3, experiment S-15). Similar comparison of experiments S-16 and S-14 shows the same correlation of general smoothness or roughness but, as in Figure 3, not always coincidence of peaks and valleys. The high initial ratios are attributed to conditions at the top of the shale bed during start-up. During this period a shallow top layer of small shale particles, heated by radiation from an electric rod, is exposed to the undiluted air flow to initiate combustion. 6000 kg Retort (0.9 m diameter, 6.1 m length) A potentially more useful application of off-gas alkene/alkane ratios is illustrated by data from the 6000 kg retort. The shale charge, consisting of both crushed material and blocks, and operating data are described in Table 2. The erratic behaviour of both ratios, indicating the roughness and complexity of this retorting operation, are shown in Figure 4. Even greater extremes in ratio values were observed in gas sgmpled inside the retort during operation, the ethene/ethane ratio reaching 30 in samples taken near a block (30 x 30 x 20 cm; 43 kg) at the 2.1 m bed depth. High ratios accompanied rapid, localized heating in various ‘hot spot’ areas as identified by thermocouples and pockets of fused shale observed after the experiment was completed. Even though these intense heat sources may not have represented overall retort

ratios: J. H. Raley

r

Gas

_--

-

1

Malrix ll-2.5

l1,3)

‘2

El

-1z?1

I.600)

SIDE VIEW

--

I-

b

Cr,-

Matrix :-s.l+o.zL)

Matrix

610

4

MI

S

i

TOP

VIEW

C

Liqula

recovery

/

Gas

chromatogroph

t-

Off~gas

a Figure5

Experiment L-2; thermocouple legend: 0, X, interior; 0, matrix shale. (a) and (c): gas sampling at top of each block, in matrix (Mt. M2) and adjacent dimensions in cm [ I. (b) Thermocouples associated

block surface; probes located to block (S); with each block

FUEL, 1980, Vol 59, June

421

Monitoring

oil shale retorts by off-gas alkenelalkane

the three blocks in the upper half of the shale bed, the amount of C,-C, hydrocarbons, in excess of that which would have been formed under assay conditions, corresponds to a loss of block-produced oil ranging from 15 to 23 wt x,. As discussed in the following section, this degradation of oil to light hydrocarbons is similar to both the thermal and oxidative pyrolysis of alkanes.

.

gO%5

,”

=04:

iz

0.2 1

I

I LO

20

0

Time

after

startup

I

I

60

80

CHEMICAL

1 h)

I

I

b @~@$ ’

I

I

I

Figure 6 Expt. L-2; relation between offgas ethenelethane ratio and retorting of blocks; - - - -, time interval of oil generation from blocks; O2 breakthrough time, in matrix (U,, M,); adjacent to block (S)

Table 3 Temperatures (6000 kg retort)

encountered

at block centre

Temperature

(“C)

Block Centre

Block surface (Avg of 4)

Matrix, 1.9 cm from block (Avg of 4)

Block

Diameter

by oil formed

No.

(cm)

1

15

325 300

570 770

760 890

2

30

325 500

690 770

850 780

3

23

325 500

570 700

750 780

4

23

325 500

480 660

640 700

5

30

325 500

580 690

740 780

6

15

325

390

450

500

580

610

vironment. The arrows show the time when oxygen (w 1 vol %) first appeared at the bed depth corresponding to the top of each block (‘oxygen breakthrough’). The arrows marked ‘s’ refer to oxygen detection adjacent to the block surface, 1.3 cm laterally from the block top edge (Figure SC). Those marked M, and M, refer to detection in the matrix 15-23 cm laterally from this edge. In all four cases where S measurements were made, oxygen was present at the block top surface during all or part of the oil evolution period. Similarly, oxygen was present in the matrix throughout the entire period of block oil production. In view of the surface and matrix temperature levels during these periods, oxidative degradation of oil is expected. Whether or not involving oxygen, the formation of lower-molecular-weight hydrocarbons is a significant route of block oil degradation. Figure 7 shows that the C,-C, hydrocarbon content of the off-gas increases substantially during the periods of oil evolution from the blocks. The C,-C, peaks coincide in time with the ethene/ethane peaks and also scale with block size. For

422

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ratios: J. H. Raley

INTERPRETATION

Chemical interpretation of alkene/alkane ratios from oil shale retorting can be inferred from the pyrolysis of pure straight-chain alkanes. Straight-chain alkanes and alkenes are important products of oil shale thermal decomposition.5 Furthermore, these products must arise mainly from straight chain moieties, whether or not attached to other structures, since thermal skeletal rearrangements are unlikely and the associated mineral components are not acidic catalysts. Data reported by Voge and Good6 for the vapour phase pyrolysis of straight-chain hexadecane over quartz chips in a flow system are summarized in Table 4. Pyrolysis at atmospheric pressure and 500°C produced C, and C, alkene/alkane ratios, independent of a two-fold change in residence time, much higher than those observed by Campbell et al4 for gas evolved from powdered shale heated to 500°C. The alkene/alkane ratio for the (C, + C,) product also is much greater than that measured by Coburn et aI.’ for C, material produced from powdered shale (ratio 0.310.75 depending on heating rate). However, an increase in pressure to 2.1 MPa (21 atm) greatly reduces all alkene/alkane ratios from hexadecane pyrolysis to values more comparable to those obtained from small particles of oil shale. The pressure increase also shifts the product distribution toward higher molecular weight, raising the liquid/gas ratio and the gas molecular weight. Thus, as the concentration is increased, the pyrolysis behaviour of hexadecane is similar to that of the condensed phase, microscopically concentrated organic matter in powdered oil shale when retoited under conditions minimizing degradation of liberated oil. However, combustion retorting of large shale S3.O 0

I

n

E I

‘-1.5’ u 0.81

I

I

I

I

I

I

al

I

”

I

I

I

I

I

I

I

I 60

I 80

I

LO lime

Ihl

Figure 7 Expt. L-2; (a) offgas C,-C, hydrocarbons concentration (excluding steam content); (b) offgas ethenelethane ratio

Monitoring Table 4 Vapour

oil shale retorts by off-gas alkenelalkane

phase pyrolysis of hexadecanee

Charge

The free radicals produced in (1) or (2) may cleave at the R-position to form an alkene and another radical (equations 3a, 3b).

Hexadecane

Temperature

(“C)

500 1 42.4

Pressure fatm) Conversiona (%I Products,

ratios: J. H. Raley

500 21 47.5

.c-CC-c-c-c--+C

c-c-c-c-

=c + Ethene

(34

wt % of charge

PC-C-C-CC-C-CC-C-CC-C-H-+ Gas (Hz + C1 -C,) Liquid, b.p. < b.p. of charge Coke Loss

17.9 23.2 1.3 0.0

13.8 32.3 0.9 1 .l

(II) c = c-c-c-c-c Alkene

+ - c-c-c-c~c--c~

(3b)

Alkene/alkane 1 .48 3.58 6.09 14.8 13 _

CZ c3 c4 CS c6

+ c7

C8

Mol/lOO 7

If the radical is primary, two additional options are available ~ intermolecular H-atom transfer to form an alkane and propagate the chain reaction (equation 4) or intramolecular transfer to form a secondary radical (equation 5).

0.392 0.742 1 .38 1 .77 2.3 2.5

mol charge converted

C2H4 C2H6 C3H6 C3’-‘, C4W3 C4”10

a Conversion the charge

17.4 23.3 14.9 38.0 27.1 36.5 19.1 13.8

-C-C-C-C-C-C. primary

-c-.c-c-c-c-c-c-c-c-c-c-c(1) Reaction initiation occurs by C-C bond rupture or via attack by a reactive species such as 0,. In either case, one or-more free radicals (designated by .) is formed. C-C Bond rupture:

(1)

Attack: (I) + reactive

-H species __t

-c-c-cC-c-cC-c-c-c-c-c-

(2) II

+-CC-C-CC-CH secondary

+

(5)

PC-C-C-C-C-C-H secondary

particles can produce local C, and C3 alkene/alkane ratios even larger than those from hexadecane pyrolysis at one atmosphere and 500°C. Ethene/ethane values measured at the top surface of the L-2 blocks in Figure 6 ranged up to 4.6 and, as noted above, reached 30 near a large block in experiment L-l. Such values are attributed to thermal and/or oxidative degradation of oil (vapour and/or entrained liquid) rather than decomposition of the condensed phase, parent organic matter. In view of the similarities between the two systems, the free-radical chain reaction scheme for alkane pyrolysis? has been applied to the thermal decomposition of the alkane moieties in oil shale and of the resultant oil. These moieties are represented, without showing the hydrogen atoms, as:

+ . c-c-c-c-c-c-

+ (II) (4)

to coke, gas, and liquid of lower boiling point than

(I)+ -c-c-c-cX-c~

+ (I)+CC~C--CC-CH Alkane

-CC-C-CC-C. 16.5 50.9 84.0 56.6 59.0 16.5 20.1 3.3

Hz ‘3

The radical cleavage typified by (3a) applies to all primary radicals except ethyl (C-C .) and is not as favourable energetically with propyl (CC-C .) as with larger primary radicals. Consequently, a major fate of ethyl radicals and an important fate of propyl radicals, is to form ethane or propane, respectively, by intermolecular H-atom transfer (equation 4). The effects of increasing pressure in alkane pyrolysis are mainly owing to the increasing importance of the second order, intermolecular transfer process (4) relative to the first order cleavage reaction (3a). Process (4) interrupts the progression of the cleavage step to sequentially smaller radicals and converts the radicals to alkanes. The products are of higher average molecular weight and the production of ethene is reduced. Similarly, propene formation, which results from intramolecular transfer followed by radical cleavage, is reduced, whereas propane formation is enhanced by increased participation of propyl radicals in reaction (4). In the thermal retorting of powdered oil shale,497 the high molecular density within the condensed phase of organic material allows the second order process (4) to assume major importance. The effect of heating rate on alkene/alkane ratios from powdered oil shale can also be interpreted in terms of (3a) and (4). As the heating rate is raised, a greater fraction of the retorting occurs at a higher temperature. The activation energies of (4) and (3a) are estimated to be about 42 and 1255146 kJ mall’ (10 and 30-35 kcal mol-‘) respectively.* Consequently (3a), which produces ethene, or its counterpart which produces propene, increases more rapidly with heating rate than the intermolecular transfer process typified by (4). A similar argument applies to block-generated oil as it encounters higher temperatures during passage to either the block surface or the matrix environment.

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Monitoring

oil shale retorts by off-gas alkenefalkane

There is also a similarity between the thermal decomposition of the alkane moieties in oil shale and the pyrolysis of polyethylene. Kiran and Gillham’ pyrolysed low density polyethylene (p = 0.925; A,,, .= 140000; M,= 16000; 13 methyl groups per 1000 C atoms) in flowing helium at 20°C min- ’ heating rate from room temperature to 600°C. The products volatilizing between 300 and 600°C consisted almost exclusively of normal alkene/alkane pairs of the same carbon number. For the C, -C,, components, the alkene/alkane values were higher than those from powdered shale’, perhaps owing in part to the high (2OC min- ‘) heating rate used for the polyethylene decomposition. Kiran and Gillham interpreted their results using a free-radical scheme closely similar to that outlined above. As noted earlier, high alkene/alkane values during combustion retorting may reflect oil degradation by oxidative as well as thermal cracking paths. Robillard, noted a continuous rise in C,&, Siggia and Uden” alkene/alkane values from rapidly heated (40’ C min- ’ to 6OO”C), powdered shale as the flowing gas atmosphere was changed from helium to 10 vol y0 oxygen in helium. Absolute amounts of both alkene and alkane increased as the oxygen content of the flowing gas was but decreased with further rise in raised to 5 vol Y
ratios: J. H. Raley bustion retort. Studies continuing at the Lawrence Livermore Laboratory are directed toward elucidating the degradation mechanisms of block-generated oil in combustion retorting of shale and the relations to composition of the retort gas product. ACKNOWLEDGEMENT J. E. Clarkson, V. L. Duval, and R. L. Ward developed the chromatographic gas analysis methods and supplied the retort gas composition data. Appreciation is also expressed to F. J. Ackerman, T. R. Galloway, R. G. Mallon, W. A. Sandholtz, and T. C. Erven for their cooperation in the design, execution, and data acquisition for the pilot retort experiments.

REFERENCES 1

2

3

4 5

6 1

2C,H,,+,

+G, +2C,H,,

+ 2H,O

have been discussed in detail by Knox.” The initial stage of both ethane and propane oxidations produces the corresponding alkene in high yield, and higher alkanes also form substantial amounts of alkenes.” Oxidative cracking and dehydrogenation may thus be significant in the oxygen-limited zone between oilgeneration and combustion regimes in a shale com-

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8

9

10 II

Raley, J. H., Sandholtz, W. A. and Ackerman, F. J. ‘Results from Simulated, Modified In Situ Retorting in Pilot Retorts,’ 11 th Oil Shale Symposium Proceedings, Colorado School of Mines Press, Golden, Colorado, 1978, p. 331 Jacobson, I. A., Jr., Decora, A. W., and Cook, G. L. in ‘Science and Technoloev of Oil Shale.’ (Ed. T. F. Yen) Ann Arbor Science Publish&, Ann Arbor, Michigan, 1976, p. 103 Sandholtz, W. A. and Ackerman, F. J. ‘Operating Laboratory Oil Shale Retorts in an In-Situ Mode,’ Lawrence Livermore Laboratorv Renort. UCRL-79035. August 18. 1977 Campbell,-J. H:, Koskinas, G. H., Galkgos, G. and Gregg, M., submitted to Fuel Coburn. T. T. and Campbell, J. H. ‘Oil Shale Retorting: Part 2. Variation in Product Oil Chemistry During Retorting of an Oil Shale Block,’ Lawrence Livermore Laboratory Report, UCRL-52256, Part 2, Sept. 8, 1977, and references cited therein Voge, H. H. and Good, G. M. J. Am. Chem. Sot. 1949, 71, 593 Coburn, T. T., Bozak, R. E., Clarkson, J. E. and Campbell, J. H. Anal. Chem. 1978, SO, 958 Laidler, K. J. and Loucks, L. T. ‘Comprehensive Chemical Kinetics,’ (Eds. C. H. Bamford and C. F. H. Tipper) Vol. 5, American Elsevier Publishing Co., Inc., New York, 1972, p. 59 Kiran, E. and Gillham, J. K. J. Appl. Polymer Sci. 1976, 20, 2045 Robillard, M. V., Siggia, S. and Uden, P. C. Anal. Chrm. in press Knox, J. H. in ‘Oxidation of Organic Compounds, Vol. II, Advances in Chemistry Series’, (Ed. R. F. Gould) Vol. 76, Am. Chem. Sot. 1968, p. 1