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Chemistry and Technology of Fuels and Oils, Vol. 48, No. 4, September, 2012 (Russian Original No. 4, July-August, 2012)

RESEARCH

THERMODYNAMIC PARAMETERS OF CONVERSION REACTIONS OF SOME

HEAVY OIL COMPONENTS UNDER THE ACTION OF STEAM AND HEAT

V. A. Lyubimenko, N. N. Petrukhina, B. P. Tumanyan,

and I. M. Kolesnikov

____________________________________________________________________________________________________

I. M. Gubkin Russian State University of Oil and Gas. Translated fromKhimiya i Tekhnologiya Topl iv i

Masel, No. 4, pp. 27 33, July August, 2012.

0009-3092/12/48040292 2012 Springer Science+Business Media, Inc.

The mechanism of the reactions of conversion of heavy oil components, viz., heteroatomic compounds

and polycyclic aromatic hydrocarbons, under conditions of steam and thermal action on the oil reservoir

is studied. Based on the calculation of the thermodynamic parameters of the reactions, conclusions are

drawn regarding the feasibility of the reactions and the primary directions of conversion of heavy oil

components at the steam and thermal action temperature. The possibility, in principle, of occurrence of

hydrogenolysis, hydrogena tion, and hydrocracking in the presence of such hydrogen donors as polycyclic

naphthenic-aromatic hydrocarbons and formic acid in the reaction system is demonstrated.

Key words: heavy oil, native asphalts, steam and thermal action, steam and gravity drainage, Gibbs

energy, aquathermolysis, hydrolysis, hydrogen donor.

Russ ia posse sse s subs tan t i a l r e se rves o f na t ive a spha l t s - va r ious ly p red ic t ed to r ange

from 30 to 75 billion tons [1], of which the recoverable reserves, even at very low recovery factors, exceed 1 billion

tons [2]. More than 500 heavy crude oil deposits are concentrated in the Volga-Ural oil- and gas-bearing

province [3] . The to ta l recoverable reserves of heavy crude o i ls in th is province comprise more

than 660 million tons. Note that the major part (97%) of these oils is high-sulfur.

These deposits cannot be recovered by traditional means, which makes search for economically efficient

techniques of heavy oil and native asphalt recovery an urgent task. Steam-thermal action (STA) on the reservoir,

such as cyclic, area, steam-gravity drainage, etc., is well studied and used widely [4]. Steam injected into the

reservoir is not only an effective heat carrier and a displacing agent, but also leads to chemical conversion of the

crude oil. This is confirmed by comparative studies of the physicochemical properties of native oil and the oil

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recovered by STA [5]: these specimens differ in density, fractional and group composition, IR spectral

characteristics, content of heteroatoms, etc.

In modeling STA in a flow-type reactor [6], resin-asphaltene components, in particular, were shown to

undergo transformation with increase in oil content in the transformed material. Modeling of hydrothermal

transformations of asphaltenes of native asphalt in hydrogen and steam helped detect considerable increase in

radical paramagnetism [7]. The content in the aqueous phase of products of reaction of hydrocarbons and

oxygen-containing compounds as well as redistribution of group components of native asphalt upon hydrothermal

transformation makes i t possible to suggest that asphaltene degradation occurs at a lkyl substi tuents

containing carbon-heteroatom bonds [6-8]. This is corroborated by slightly lower values of the energy of

the C-S and C-O bonds in sulfides and ethers than the energy of the C-C bond [9].

In [10, 11], modeling of STA indicated steep rise in volume of carbon dioxide and hydrogen sulfide liberated

at temperatures above 200C. It is suggested that hydrogen sulfide is formed upon thermal decomposition of

organosulfur compounds, reaction between sulfur and paraffins with formation of mercaptans and decomposition

of the latter. Steam was shown to have a catalytic effect on carbon dioxide generation. Catalytic effect of water on

transformation of organic compounds is reported also in [12]. With rise of temperature the solubility of

organic compounds in water increases and, moreover, at 250C the negative logarithm of the ionic product of water

is 11 (at 20C it is 14). So with rise of temperature water simultaneously becomes a stronger acid and a stronger

base , catalyzing reactions of hydrolysis of some heteroatomic compounds. As shown in [13], in STA water may

not only be a catalyzer but also a reagent. Analysis of thiophane and thiophene transformation products under

conditions of thermolys is in steam [13] showed relatively high carbon monoxide and carbon dioxide content in the

reaction gas, although the initial sulfur compounds do not seem to contain oxygen, i.e., water is the only source

of oxygen in the system.

Summing up the foregoing it can be suggested that STA may stimulate reactions of thermal cracking of

hydrocarbons and heteroatomic components, high-temperature hydrolysis (aquathermolysis) of heteroatomic

compounds [14], condensation, hydrogen migration, i.e., dehydrogenation of some compounds simultaneously

with saturation and hydrogenolysis of others. It is of interest to determine the thermodynamic feasibility ofoccurrence of these reactions of transformation of some components of heavy crudes at temperatures typical

for STA. For this purpose, the thermodynamic parameters (fH

T and S

T) of the reactants considered as model

compounds of heavy crudes were calculated by the roentgenometric method PM6 using the MOPAC 2007 software

package and the change in Gibbs energy of the react ions was calculated by the equat ion rG

T =

rH

T - T

rS

T.

Transformation of organosulfur compounds. Hydrolysis of carbon-heteroatom bonds is stimulated by

part ial negative charge on the heteroatom, i.e. , by its abil ity to bind a proton (protonate), so hydrolysis intensif ies

in acidic medium. The following mechanism of high-temperature hydrolysis of cyclohexylphenyl sulfide can be

suggested [15]:

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or in the shortened form

(1)

(2)

(3)

For cyclic sulfides, particularly for thiophane, the following transformation chain was suggested in [13]:

To check the feasibility of the chain of transformations, the thermodynamic parameters were calculated for

the first stage:

(4)

Since hydrogen sulfide formed upon STA is a weak acid, it is capable of protonating heteroatomic

compounds of crude oil and initiating their hydrolysis, i.e., autocatalytic reactions may occur under the action

of H2S [13].

The initial reagents noted above may be a part of resin molecules and asphaltene structures. The following

aquathermolysis reaction at the CS bond can be written for the model of the molecule of resinous matter:

(5)

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As can be seen, destruction of C-S bonds may cause detachment of individual fragments from the resin

and asphaltene particles. Note that the chains containing heteroatoms bind aromatic plates into asphaltene

packets [16]. Upon breakup of carbon-heteroatom bonds, alongside a change in the group and fractional

composition, the structure of the asphaltene particles may undergo transformation, causing, in turn, a change in

the rheological properties of the material.

Apparently, at temperatures between 200 and 350C, reactions of thermal cracking of sulfur-containing

compounds will occur with formation of hydrogen sulfide, hydrocarbons, condensation products, etc.:

1262136136 H2CSHHCSHC (6)

In some works, [17, 18] in particular, use of hydrogen donors for accelerating transformation of heavy oil

components by STA is proposed. The most common hydrogen donor is tetralin (tetrahydronaphthalene), which is

amenable to dehydrogenation with liberation of active hydrogen:

(7)

Decomposition of formic acid may also be a source of hydrogen:

22 COHHCOOH (8)

But, according to [19], in gas phase formic acid undergoes decomposition by the reaction:

COOHHCOOH 2

(9)

Fig. 1. Dependence of change in Gibbs energy of hydrogen formation reaction on

temperature (the digits on the curves correspond to reaction numbers in the text).

Gib

bsenergy,

kJ/mole

Temperature, K

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In liquid phase, on the other hand, formic acid forms with water complexes that decompose to hydrogen

and carbon dioxide; this reaction is catalyzed by water, on the concentration of which depends the acid

decomposition rate [19]. Reaction between steam and carbon dioxide - a product of decarboxylation of oxygen-

containing organic compounds, may also be another source of hydrogen in the system:

222 HCOOHCO (10)

In order ascertain the thermodynamic feasibility of reactions (7)-(10), which may be sources of hydrogen

in the productive stratum, their rG

T values were calculated (Fig. 1). As will be seen, tetralin hydrogenation is

possible a t temperatures above 337C, and both formic acid decomposition reactions and reaction (10) are possible

at the STA temperature.

The thiophane hydrogenolysis reaction in the presence of tetralin as the hydrogen donor can be written

a s

(11)

Upon dissociation of formic acid to the ions H +and HCOO-, the latter will react with the organosulfur

compounds following the scheme [20]:

or in the shortened form

(12)

A similar reaction for thiophane is:

(13)

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The results of calculation of the Gibbs energy of reactions (1) (6) and (11)(13) at various

temperatures are plotted in Fig. 2a . The positive rGT values of cyclohexylphenyl sulfide hydrolysis

reactions (1) and (2) attest to occurrence of these reactions with low yield of products; the equilibrium is shifted

toward the initial reagents. Reaction (1) is somewhat more probable than reaction (2). At the same time,

decomposition of cyclohexylphenyl sulfide to thiophenol and cyclopentane (3) is possible at temperatures

above 227C. Hydrolysis of the CS bond in the model resin compound (5) is thermodynamically feasible in the

whole STA temperature range, so transformation of resins and asphaltenes with detachment of side fragments and

formation of hydrocarbons and heteroatomic components with a lower molecular weight is possible. The

calculated rG

Tvalues for the first stage of thiophane transformation (4) are positive, which indicates that this

reaction is improbable under the STA conditions. From a comparison of reactions (1), (4), and (5) it is obvious that

as the structure becomes more complex and the molecular weight of the organosulfur compounds increases, the

probabil ity of occurrence of hydrolysis reactions increases. Hydrolysis wi th ring sc ission , on the other hand, is

highly improbable.

Fig. 2. Dependence of change in Gibbs energy of reactions of transformation of heavy

oil componen ts by STA on temperatu re: a- organosulfur compounds; b- organooxygen

and organonitrogen compounds; c - PCAH (the digits on the curves correspond to

reaction numbers in the text).

Gibbsenergy,

kJ/mole

Temperature, K

c

b

a

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Calculations show that thermal degradation of dihexyl sulfide (6) is possible at temperatures not

below 227C. It is interesting to note that reactions (12) and (13) of sulfur compound hydrolysis in the presence

of formic acid may occur in the whole investigated temperature range, whereas thiophane hydrogenolysis reaction

in the presence of tetralin (11) may occur at temperatures above 120C, i.e., it is also possible under STA.

Note that in th is work tetral in is considered as one of the polycycl ic naphthenic aromatic crude oil

hydrocarbons that exhibit hydrogen donor properties. Thus, under STA hydrogen will be transferred from these

hydrocarbons to other components, heteroatomic in particular, and the latter will undergo hydrogenolysis.

Transformation of oxygen- and nitrogen-containing organic compounds. The structure of resins and

asphaltenes contain ROR2 groups whose hydrolysis will probably occur with formation of phenols [20]:

(14)

For the model of the molecule of resinous matter, referred to above, we may write two hydrolysis reactions

occurring with breakup of only the CO bond of aliphatic ether and breakup of CO bonds of both aliphatic and

cyclic ether:

(15), (16)

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Akin to reactions (1) and (3), reactions of phenylcyclohexylamine hydrolysis and degradation may occur

as follows [15]:

(17), (18)

In the presence of formic acid, aniline and cyclohexane may form:

(19)

The results of calculation of the change in Gibbs energy of reactions (14)(19) at various temperatures are

plot ted in Fig. 2b. As can be seen, the referred reactions of hydrolysis of oxygen-containing compounds are

possible in the whole STA tempera ture range. Thus, hydrolysis of CO bonds will occur with greater transformation

than hydrolysis of CS bonds. Hydrolysis of phenylcyclohexylamine by reaction (17) is possible only

at 230C and above, and its decomposition by reaction (18), at a temperature above 130C. Transformation of

nitrogen-containing compounds in the presence of formic acid, like oxygen-containing compounds, is

thermodynamically possible in the whole investigated temperature range.

Transformation of polycyclic aromatic hydrocarbons (PCAH). Calculation of thermodynamic characteristics

of PCAH transformation reactions under STA conditions is of interest from the standpoint of determination of the

most probable paths of their transformation: condensation, hydrogenation, hydrogen rearrangement, and breakup

of CC bonds joining the rings. The latter reaction helps get an idea of the possibility of detachment of aromatic

fragments from resins and asphaltenes as a result of breakup of the CC bond rather than the carbonheteroatom

bond as was shown earl ier. Let u s examine the t rinaphthyl transformation reactions.

Condensation:

(20)

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Hydrolysis in the presence of hydrogen donors:

(21), (22)

For phenanthrene, we may suggest hydrogenolysis reaction in the presence of hydrogen donors in the

system:

(23), (24)

The results of calculation of Gibbs energy of reactions (20)-(24) at various temperatures are plotted

in Fig. 2c. It is quite predictable that from the point of thermodynamics condensation reaction (20) may occur in

the whole investigated temperature range. Reactions of PCAH hydrogenolysis in the presence of formic acid as

the hydrogen donor many also occur in the whole STA temperature range. This cannot be said, however, about

polycycl ic naphthenic aromat ic hydrogen donors . Thus, reaction (21) of hydrogen transfer from te tral in to

trinaphthyl is possible at a temperature higher than 227C, but reaction (24) of hydrogen transfer from tetralin to

phenanthrene is not poss ible at al l.

Based on the results of the performed calculations it can be concluded that the following chemical

transformations are most likely when heavy crude oils and native asphalts are recovered by steam-thermal technique:

hydrolysis of ether, sulfide, and amine bridges with detachment of lower-molecular fragments such as

paraffinic, aromatic, he teroatomic, etc.;

thermal cracking of heteroatomic components and PCAH, including with formation of gases,

lower-molecular components, and condensation products;

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transfer of hydrogen from polycyclic naphthenic aromatic hydrocarbons to organic sulfur compounds

and PCAH with formation of hydrogen sulfide and lower-molecular products, respectively;

hydrogenolysis of heteroatomic compounds and hydrocracking of PCAH with involvement of the hydrogen

formed in the reaction of carbon monoxide conversion with steam.

When formic acid is used as the hydrogen donating additive, hydrolysis of nitrogen- and sulfur-containing

compounds and hydrocracking and hydrogenation of PCAH may also occur.

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