Kinetics and Energetics of Oxidation of Bitumen in Relation to In Situ

Chapter 23

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Kinetics and Energetics of Oxidation of Bitumen in Relation to In Situ Combustion Processes Leslie Barta, Andrew W. Hakin, and Loren G. Hepler Department of Chemistry, University of Alberta, Edmonton, Alberta T6G 2G2, Canada Using a "home made" aneroid calorimeter, we have measured rates of production of heat and thence rates of oxidation of Athabasca bitumen under nearly isothermal conditions i n the temperature range 155-320°C. Results of these k i n e t i c measure ments, supported by chemical analyses, mass balances, and fuel-energy relationships, indicate that there are two p r i n c i p a l classes of oxidation reactions i n the specified temperature region. At temperatures much lower than 285°C, the p r i n c i p a l reactions of oxygen with Athabasca bitumen lead to deposition of " f u e l " or coke. At temperatures much higher than 285°C, the p r i n c i p a l oxidation reactions lead to formation of carbon oxides and water. We have f i t t e d an o v e r a l l mathematical model (related to the f a c t o r i a l design of the experiments) to the k i n e t i c results, and have also developed a "two reaction chemical model". Subsequent measurements i n which water vapor has been introduced along with oxygen have led to modified k i n e t i c s and also a modified chemical model for wet oxidation of Athabasca bitumen. In s i t u combustion processes involve i n j e c t i o n of a i r or oxygen i n t o the formation to burn part of the bitumen, thereby producing heat that raises the temperature of the reservoir. At low tempera tures oxidation i s incomplete and leads to formation of " f u e l " or partly oxidized bitumen that has higher v i s c o s i t y and lower heating value than the o r i g i n a l bitumen. These incomplete oxidation r e actions, c o l l e c t i v e l y called "low temperature oxidation" or "LTO", predominate during the i g n i t i o n delay period of i n s i t u combustion and also occur ahead of the combustion front when oxygen i s a v a i l able. I t i s therefore important to have knowledge of the kinetics and energetics of oxidation of bitumen i n the temperature range i n which LTO occurs. 0097-6156/89/0396-0426$06.00/0 © 1989 American Chemical Society In Oil-Field Chemistry; Borchardt, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

23. BARTAETAL.

All

Oxidation ofBitumen

In this paper we summarize some of the results of our measure ments of rates of dry oxidation. Results of chemical analyses of residues produced by heating i n flowing nitrogen atmosphere ( d i s t i l l a t i o n ) are also reported and combined with our k i n e t i c data to obtain values of k i n e t i c parameters. Preliminary r e s u l t s of measurements of rates of wet oxidation are presented.


Experimental The focus of our investigations of the k i n e t i c s of oxidation of Athabasca bitumen has been on the use of an aneroid calorimeter (_1_) for measuring rates of heat production under nearly isothermal (AT < 1.2°C i n each experiment) conditions. I n i t i a l attention was given to just two of the variables that a f f e c t the k i n e t i c s of oxidation: (i) temperature and ( i i ) pressure of oxygen. Preliminary studies into a t h i r d variable, the p a r t i a l pressure of water vapor i n the system, are discussed i n Part 3 of the Results and Calculations section. Each calorimetric sample (»1 g, 13.47 mass % bitumen) came from a large sample of "reconstructed" o i l sand consisting of Athabasca bitumen loaded onto a chemically i n e r t s o l i d support material (60/80 mesh acid washed Chromosorb W) of well-defined p a r t i c l e s i z e . We represent the o v e r a l l oxidation reaction by Bitumen + 0

2

•> chemical products + heat

(1)

and express the rate of reaction i n terms of the rate of production of heat as i n Rate = dQ/dt = k [ p ( 0 ) ]

r

2

(2)

where Q i s the instantaneous heat produced per gram of bitumen, t i s time, p(02) i s the pressure of oxygen, k i s a s p e c i f i c rate constant, and r i s the reaction order with respect to pressure of oxygen. Because the stoichiometry of the process represented by Equation 1 i s not known and because f u e l q u a l i t y changes with extent of reaction, we have used the method of i n i t i a l rates f o r evaluation of r and k. Elemental compositions and masses of organic residues produced by d i s t i l l a t i o n and by oxidation were determined as described previously (2_). Results and Calculations Part 1. Kinetics and Energetics of Dry Oxidation. The simplest approach to data analysis i s to assume that only a single class of oxidation reactions i s important and to make the related assumption that the temperature dependence of the single rate constant k can be represented by an Arrhenius equation. In this way we obtain In W. = In A - E /RT + r In [ p t O ^ ] a

(3)

i n which W i s the i n i t i a l rate of heat production, A i s the Arrhenius pre-exponential factor, and E i s the average a c t i v a t i o n i

a

In Oil-Field Chemistry; Borchardt, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

428

OIL-FIELD CHEMISTRY

energy. According to this simple model, both E and r are independent of temperature. Consideration of a f a c t o r i a l design model showed that we require the results of five experiments (different temperatures and pressures of oxygen) to test Equation 3. Results of such experiments and related mathematical analysis (_2) have shown that the " f i r s t order mathematical model" or "one class of reaction chemical model" represented by Equation 3 i s inadequate. Because e a r l i e r experimental results and data analyses (3-10) had led us to anticipate the inadequacy of the simple approach considered above, we also planned and carried out (2) a second order f a c t o r i a l design of experiments and related data analysis. Mathematical analysis (of the results of 11 experiments) based on the second order model showed that a l l of these results could be represented s a t i s f a c t o r i l y by an equation of the form


a

In w

±

= b

Q

+ (b +b /T)(1/T) + (b +b /T) In [ p ( 0 ) ] 1

3

2

4

2

i

(4)

Comparison of Equation 4 with Equation 3 shows that In k = b

+ (b +b /T)(1/T)

Q

3

(5)

+ b /T

(6)

1

and r = b

2

4

Because the dependence of k i n e t i c parameters k and r on temperature as described by Equations 5 and 6 i s constrained by the f a c t o r i a l design, i t i s possible that these equations and therefore Equation 4 may not give the best o v e r a l l representation of the k i n e t i c data. We have therefore carried out four more experiments and analyzed these results along with those mentioned e a r l i e r to obtain (2) the best o v e r a l l mathematical representation that i s summarized by the following equations: In k = a

Q

r = a In W

t

= a a

+ a ^ + a /(595-T) 3

4

2

(8)

4

+ a-T + a /(595-T) + a In [p(O ) ]/(600-T) 2

(7)

+ a /(600-T) 2

Q

2

2

3

In [ p ( 0 ) ] + 2

i

i

(9)

The composite rate constant k summarized by Equation 7 has a maximum near 302°C. The reaction order r i s nearly constant at low temperatures and increases dramatically at higher temperatures. The k i n e t i c results and related analysis (2) summarized above indicate that there i s a change i n the predominant class of oxida tion reaction with increasing temperature, which led to the expectation that the t o t a l heat developed i n the o v e r a l l oxidation also depends on temperature. Because the measurements that led to k i n e t i c data based on i n i t i a l rates were continued nearly i s o thermally u n t i l oxidation was complete, i t has also been possible to establish (2_) that the t o t a l heat developed increased by nearly ten-fold over the range 155 to 320°C.


Oxidation ofBitumen

23. BARTA ET AL.

429


On the basis of our k i n e t i c results, our heats of oxidation mentioned above, independent heats of t o t a l combustion (Yan, H-k.; Hepler, L.G. to be published), our chemical analyses, and the results of e a r l i e r investigations by others (11-15), we have developed a chemical model or picture of the dry oxidation process as follows. We propose that the complicated dry oxidation of bitumen can be represented as the sum of contributions from two classes of oxidation reaction. One class of reactions i s the p a r t i a l oxida tion that leads to deposition of coke and formation of "oxygenated bitumen", with very l i t t l e production of carbon oxides and water. This class of reactions i s concisely summarized by Bitumen + 0

2

Coke + (other chemical products) + Qq

CO)

in which Q represents the heat production associated with deposi t i o n of f u e l . The second class of reactions i s similar to conventional combustion or burning reactions that y i e l d mostly carbon oxides and water vapor, as summarized by D

Bitumen + Fuel + 0

> Carbon Oxides + Water Vapor + Qq

2

(11)

in which Qg represents the heat produced by the "burning reactions"• The "two classes of chemical reactions model" represented by Equations 10 and 11 can be represented mathematically by the following:

ln W

= ln A

ln W

= In A

D

B

D

- E

a ( D )

/RT + r

D

l n [p(0 )]

(12)

fi

- E

a ( B )

/RT + r

B

l n [p(0 )]

(13)

2

2

Not a l l of the bitumen i n i t i a l l y present i n the calorimetric sample i s available for oxidation, due to d i s t i l l a t i o n during heating to the temperature of the experiment. As a preliminary to evaluating the k i n e t i c parameters appearing i n Equations 12 and 13, we have performed d i s t i l l a t i o n experiments as described i n Part 2 to i d e n t i f y the mass f r a c t i o n of the o r i g i n a l bitumen that i s available for oxidation a t each temperature. Kinetic parameters based on the i n i t i a l rate of heat production per gram of bitumen available are l i s t e d i n Table I. The temperature a t which deposi tion and burning contribute equally to the t o t a l rate of heat production i s ~270°C. Deposition continues to be important to a l i t t l e over 300°C. The r a t i o of the rate of heat production by burning to that by deposition i s >" 12.20 11.30 8.77 7.74 6.16 5.18

C

Bitumen Remaining 100.00 83.90 71.90 63.40 50.50 42.50

Mass % H

82.49 83.75 82.45 80.78 81.69 80.34

N

S

9.64 9.32 9.29 8.58 7.67 7.58

0.41 0.71 0.46 0.13 0.16 0.10

5.45 6.60 7.80 8.71 7.52 10.28

Loading factor (%).

Table II l i s t s the elemental compositions of the organic residues produced at the various temperatures. The molar H/C r a t i o of the residue produced by heating decreases s l i g h t l y with i n creasing temperature, from 1.3 at 155°C to 1.1 at 330°C. Carbon content of the organic residue produced by heating i s approximately independent of temperature i n the range 155-330°C. Table I I I l i s t s the mass fractions, expressed as %, of o r i g i n a l bitumen and of o r i g i n a l C, H, and S remaining as residue. The mass % of whole bitumen and mass % of o r i g i n a l carbon remaining at each temperature are approximately the same. Sulfur i s not lost by heating u n t i l the temperature exceeds 230°C, and appreciable S (more than half the o r i g i n a l mass present) remains even at 330°C. Table I I I . Mass % of Total Bitumen and of C, H, and S Remaining as Residue After Heating Temperature (°C) 25 155 182 187 229 275 330

Bitumen

C

H

S

100.0 83.9 71.9 71.9 63.4 50.5 42.5

100.0 85.2 76.2 74.9 62.1 50.0 41 .4

100.0 91.1 69.3

100.0 101 103

-

56.4 40.2 33.4


-

101 69.7 80.2

OIL-FIELD CHEMISTRY

432

No condensate was found i n the cold trap (ice or dry ice/acetone) after any of the experiments, nor was any material recovered from the gas t r a i n by flushing with toluene and ethanol. However, the quantity of condensate expected on the basis of ashing of the heated samples i s gravimetrically s i g n i f i c a n t i n a l l cases (15 mg expected at 155°C, 56 mg expected at 330°C). Table IV. Mass Balance of Carbon i n Dry Oxidation, Expressed as Mass % Relative to Original Mass of Carbon ( D i s t i l l a t i o n ) and Available Mass of Carbon (Final Products)


Distillation Temperature (°C) 155 174 225 285 317

C

C

A(s)

82.4 75.0 59.5 48.7 44.6

A(v)

C

17.6 25.0 40.5 51 .3 55.4

F i n a l Products of Oxidation Heat (kJ/g ox(v) ox(s) b i t avail) 22.7 1.9 77.3 4.0 (69.7) (30.3) 12.5 50.3 49.7 16.4 70.2 29.8 18.1 100.0 0.0 C

The mass balance f o r carbon during dry oxidation of bitumen i n our calorimeter can be calculated from the above results i n combin ation with some of our chemical results f o r oxidized samples reported elsewhere (_2_). This mass balance i s summarized i n Table IV, where the following relationships have been used: C

t o t a l = A(s) A(v)

C

+

C

A(s) " ox(s)

C

C

+

C

ox(v)

Kinetics and Energetics of Oxidation of Bitumen in Relation to In Situ

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