Chemical and Physical Processes of Hydrocarbon Combustion: Chemical Processes Rodney A. Geisbrecht and Thomas E. Daubert* Department of Chemical Engineering, The Pennsylvania State University, University Park, Pennsylvania 76802
An initial attempt at firmly establishing the interrelationship of vapor-phase partial oxidation processes to the associated process parameters has been undertaken. This has been specifically done for the oxidation of ethane in the presence or absence of a countercurrent rain of particulate solids which enable uniform and stable control of the extreme reaction heat. It has been shown that these systems are comprised of a coupled chemical and physical transport process. Part I of this two-part series deals with the chemical processes of ethane partial oxidation. A mechanism is developed on the bases of experimental data illustrating the effects of temperature, pressure, composition, and contact time in conjunction with estimated rate parameters. The mechanism has been found very effective in quantitatively correlating and predicting experimental behavior over substantial ranges of conditions using derived Arrhenius parameters which are consistent when compared with theoretical estimates.
Introduction As the world’s petroleum reserves continually diminish, efficient conversion processes for the production of necessary materials will become a critical factor affecting virtually all aspects of our society. One such process which has been the subject of long-term study a t this laboratory is the partial oxidation of hydrocarbons in the vapor phase. The initial phase of this long-term study, begun over ten years ago, concerned itself primarily with the general characteristics of partial oxidations such as typical conversion levels. I t soon became apparent that standard reactor systems were essentially unsuitable for partial oxidation processes primarily because of their inability to control the exothermicity of these gas phase processes efficiently. Work was then directed toward the development of a suitable reactor system culminating in the development of the raining solids reactor (Jones and Fenske, 1959). This type of reactor system utilizes a countercurrent “rain” of relatively fine particulate solids in what would otherwise be an open tubular reactor. The particles absorb excess thermal energy, thereby retarding hot spot formation and resulting in suitable and stable process operation. Oxidant injection is staged along the reaction zone primarily to further retard hot spot formation but also to assure that operating levels always remain beyond those which are conducive to explosive processes. Subsequent to the development of the raining solids reactors extensive experimental research was initiated to establish the products and conversion levels obtainable under various operating conditions utilizing various hydrocarbons ranging from paraffins to aromatics (Jones, et al., 1961a,b, 1970, 1971a,b). Experiments were also undertaken to establish the utility of these processes for various conversions presently being employed in the petroleum and petrochemical industries as well as the chemical industry. Applications such as gas-oil conversion to more desirable “cuts” and gasoline upgrading have been specifically studied (Daubert, et al., 1963; Jones, et al., 1971). A t present this laboratory is considering a study concerning the applicability of partial oxidation processes displacing many of the current endothermic processes such as catalytic/thermal cracking from an energy conservation standpoint. Experimental studies such as those mentioned above have clearly demonstrated that the utility of partial oxidation processes critically depends upon the influences of the various independent process parameters. Studies have therefore been undertaken to establish the interrelationship of the partial oxidation processes to the process parameters.
For ease of presentation, this work will be presented in a two-part series. This paper will deal with the chemical process of hydrocarbon partial oxidation systems while a companion paper will consider the associated physical processes which have been found to be of extensive significance in combustion systems. Experimental Section Apparatus and Procedure. A critical part of this study consisted of the experimental determination of temperature influences, in the absence of particulate solids, associated with a system in a particular reactor for which the influences had previously been determined in the presence of particulate solids. For this reason the ethane partial oxidation system in the raining solids prototype completely described by Svoboda and Daubert (1972) and Jones and Fenske (1959) was utilized. The ethane feedstock was always a t least 99.5% pure with ethylene the major impurity. Product recovery was accomplished by the following arrangement. Immediately after exiting from the reaction zone the crude product mixture encounters a direct water spray located above a shell and tube heat exchanger. Any condensable and/or water soluble products are stripped from the gas phase and are collected in a receiver packed in ice. This product constitutes what is referred to as the aqueous layer. The noncondensable and insoluble gas mixture passes through a back-pressure regulator after which the mixture is monitored by a wet test meter. A small sampling port is provided just ahead of the pressure regulator to achieve a continuous gas sampling in a constant volume sampler. Steady state, repeatable experiments of approximately 1 hr duration were made in each case. Analysis. The products were analyzed by separating oxygenated materials from water by distillation followed by gas-chromatographic analysis. Gases were analyzed by gas chromatography, formaldehyde by visible spectroscopy, and acids by titration. A complete description of the methods is given by Jones, et al. (1969a,b). Results a n d Discussion Partial oxidations can be described as complex reaction systems in that the total product is usually comprised of a mixture of many compounds. The relative distribution of these compounds within the total product, as well as the quantity of total product, is always a strong, and very often a complicated, function of the usual process parameters: (1) temperature, (2) pressure, (3) feed composition Ind. Eng. Chem., Process D e s . Dev., Vol. 14, No. 2, 1975
159
(hydrocarbon and oxidant), and (4) contact time. In addition to these usual process parameters there are others upon which the partial oxidation processes are dependent: ( 5 ) particulate solids (structure (e.g., silica gel), inlet temperature, and mesh size), and (6) reactor design (material of construction, oxidant and feed inlet placement, reactor diameter, and reactor surface to volume ratio). The existence of such a number of influential process parameters, with each parameter modifying the effects of other parameters, indicates that even a statistically welldesigned experimental strategy to cover the practical range of partial oxidation processes would of itself be impractical. This is especially true when considering the very often complicated and cumbersome experimental procedure which is required. A model or, more to the point, a mechanism of the reaction processes must be developed to correlate experimental data and to effect reliable scale-up of pilot plant data. To facilitate these needs and also to provide any usable insight into the processes occurring the mechanism must be theoretically based upon so-called elementary reactions. From the very start, however, there are certain considerations which lead one to conclude that a quantitative mechanistic study, based upon the available data in the literature, would be both fruitful and valid only if restricted at this time to the product distribution in preference to the conversion of feed hydrocarbon. For one reason, under most practical operating conditions the primary factor in determining the suitability of a given partial oxidation process is the product distribution. Also, by virtue of the relatively long kinetic chain lengths referred to later, there are certain complicating factors such as heterogeneous effects which change the conversion of feed hydrocarbon but which do not affect the product distribution. The product distribution will be considered in detail. The developed mechanism of reaction will in essence comprise the propagation steps of the total reaction mechanism of partial oxidation and will be referred to through-
out the remainder of these discussions as the carbon-path mechanism. Ethane will be used as the model feedstock mainly because of simplicity and experimental data availability. Once the mechanism of reaction and the influences of process parameters are firmly established, the extension to other organic partial oxidation processes is reasonably straightforward. Development of the Carbon-Path Mechanism. Among some of the more fundamental characteristics of hydrocarbon partial oxidations, for which there is considerable experimental evidence and agreement among researchers, is a free radical chain reaction mechanism consisting of relatively long kinetic chain lengths. However, there is considerable inconsistency between individual proposals to account qualitatively for the various products obtained during partial oxidation processes. In the specific case of ethane partial oxidation the predominant products above 200°C are ethylene, the carbon oxides, methane, methanol, formaldehyde, and acetaldehyde and smaller quantities of three and four carbon atom paraffins and olefins as well as ethanol and hydrogen. Below 200°C the alkylhydroperoxide CzH502H (ethylhydroperoxide) is formed at essentially 100% carbon efficiency. At relatively high temperatures (greater than 400°C) the olefin products are favored a t the expense of the other products, and no significant quantities of the alkylhydroperoxide C2H502H occur. The process may also reveal a region of negative temperature coefficient in the range of 350-450°C in which the feed hydrocarbon conversion decreases with temperature beyond which it once again increases with temperature. Consistent with the experimentally verified product distributions, all significant and stable reaction products have been accounted for by a series of elementary reactions which are most probable based upon their predicted activation energies, frequency factors, and the work of previous investigators. This derived carbon-path mechanism is shown in Scheme I. The detailed derivation of the
Scheme I. The Carbon-Path Mechanism
I I I I
I
I I
I I I
f
.J
, ,
O.CH,CH,O,H
I
CHO,.
I
\
CH,
+
CH?O
+
\
CHO
CO,
HO?CH,CHO~H CO
I HO,CH,CHO
+
.OH
-
.OCH,CHO I C A
HOCH,CHO 160
Ind. Eng. Chern., Process Des. Dev., Vol. 1 4 , No. 2, 1975
R
HOCH,CO
/
+ +
+
H. CH,OH p H 6
CH,OH
.OH
mechanism is available in the supplementary material in the microfilm edition of the journal. See the paragraph a t the end of the paper regarding this supplementary material. Validification of the Carbon-Path Mechanism. The derived carbon-path mechanism can now be applied to the correlation of actual experimental data using only literature and estimated values of Arrhenius parameters. The mechanism’s validity is demonstrated on the bases of derived Arrhenius parameters as well as successful correlation and prediction of experimental behavior. To facilitate the required product distribution computations based upon the carbon-path mechanism, an appropriate model of the reaction system itself must be developed. The model chosen is justifiably elementary and includes the assumptions o f (1) isothermal reactor at a temperature level equivalent to the average axial temperature; (2) plug flow reactor system; (3) negligible volume changes upon reaction; (4) differential reactor in that the product distribution corresponds to average reaction conditions. The isothermal assumption utilizing the average axial temperature is also used in reporting experimental data a t this laboratory as it is the only practical means of characterizing the temperature level in the reaction system. The plug flow assumption is one which is necessary in terms of achieving a tractable solution procedure. It does have some basis in that oxidant injection points are located along the reactor length and in that most partial oxidations occur with a rain of particulate solids which should assure sufficient turbulence to achieve plug flow conditions. Assuming negligible volume changes upon reaction is also a simplification for computational purposes. For the conditions o f (1)50% reaction by
and 50% reaction by
(2) 10% hydrocarbon feed conversion, and (3) air as oxidant with an oxygen to hydrocarbon feed mole ratio of 0.15, a volume change of about 3.5% occurs at average reaction conditions. Since the above conditions are severe with respect to volume changes, most volume changes will be less than 3.5% and will not, therefore, alter the essential nature of the calculated Arrhenius parameters. Since the conversion is usually 15% or less and since the product distribution corresponds proportionately to the conversion, one is justified in assuming that the product distribution will correspond to the average hydrocarbon feed and oxidant concentrations. All of the model assumptions except that concerning the temperature level are based upon some rationale and experimental results. The assumption concerning the temperature level is based not upon experimental results but rather upon how it can be characterized in actual practice by a finite number of temperature measurements. It will be shown in a companion paper that this model assumption is not necessarily valid under all conditions. However, it can be stated for certain that the variations of the average axial temperature will reflect the variations of the effective temperature level of the reaction system. Utilizing the derived carbon-path mechanism together with the reaction system model, appropriate expressions may be derived which specify the product distribution in terms of specific Arrhenius parameters. For example, the
alkylperoxy radical concentrations are derived in terms of the corresponding alkyl radical concentrations uia the reactive free radical steady-state approximation. The numerical designation of each specific reaction rate corresponds with the reactions list of Table I. I t should be noted that the concentrations of ethane and oxygen are calculated by means of empirically determined conversions. This offers no significant problem since for nearly all conditions of interest actual experimental data or an easily applied approximation technique consisting of a straight-line dependence of oxygen to hydrocarbon molar ratio against hydrocarbon conversion may be utilized. It should also be noted that instead of determining the yields of carbon monoxide and carbon dioxide individually, the yield of oxides (carbon selectivity) is determined. This is done because according to the carbon-path mechanism either carbon monoxide and hydrogen atom or carbon dioxide and hydroxy radical are formed so that the utilization of total oxides is valid. However, the separation of total oxides into the individual components requires the assumption of unreasonable Arrhenius parameters to account for the level of carbon dioxide formed in actual practice. Since no other reasonable route of carbon dioxide formation by homogeneous kinetics is feasible, it must be concluded that at least a large part of the carbon dioxide is formed by heterogeneous reactions. Since the total oxides are well accounted for and since variable surface experiments clearly illustrate that the CO2/CO ratio continually increases with surface area. it is concluded that heterogeneous reactions on the steel reactor or particulate solids surfaces produce some carbon dioxide from carbon monoxide. To most adequately validate the reaction mechanism quantitatively, experimentally derived Arrhenius parameters are required. This in turn necessitates experimental data of the variable temperature type. Since the effects of particulate solids are rather extensive, as will be illustrated, one experimental data set employing particulate solids and one in the absence of particulate solids are utilized. This is done because the effects of particulate solids are not inherently taken into account by a mechanism such as the one derived and also because some valuable insight into the nature of the influence of the particulate solids upon the reaction system is desired. Both data sets are taken from the same reactor and hence reactor design is accounted for when examining the particulate solids effect. The individual data sets for use of particulate solids as heat carriers and the open tube are referred to throughout the remainder of these discussions as the controlled and uncontrolled reaction system sets, respectively. The controlled data set is taken from the work of Jones, et al. (1969a), and is reproduced in Figure 1. The uncontrolled set, taken in conjunction with this work, is summarized in Figure 2. The products are reported in terms of the product families: (1) olefins-mainly ethylene; ( 2 ) O X ygenated compounds-mainly methanol; (3) carbon oxides-mainly carbon monoxide; and (4) paraffins-mainly methane. The legend shown under Figure 1 applies to all figures. Temperature Effects. Since both data sets appear to be fairly consistent within themselves, it is unlikely that the discrepancy between the individual sets, especially noticeable a t the lower temperature levels, is attributable to experimental deviations. The discrepancy must, therefore, be a result of the particulate solids influence on the reaction system. This particulate solids influence is one such that a t any given experimentally measured temperature the product distribution is shifted in favor of the lowtemperature products, oxygenated compounds and carbon oxides, a t the expense of the high-temperature products, Ind. Eng. Chem.. Process Des. Dev., Vol. 14, No. 2, 1975
161
,
*
Table I. Final Tabulation of Arrhenius Reaction Rate Parameters for Determining the Bulk Product Distribution of Ethane Partial Oxidation" Log A (English units) Reaction
Estd
1. C2H5. + 0, --+ C,H, + H02* 2 . C,H,- + 0, CZHj02. 3 . C2H,02. CzH5. + 0, 4 . C2H,02. + C2HC + C2HjO2H + CzH5. 5 . C2H5O2CH,CHO + .OH 6. C , H 5 0 2 - .--t .CH,CH,O,H 7 . -CH,CH,O,H --+
-
-
ZH50?'
8. C H 2 C H 2 0 , H
-
C,Hj + HO,. 9 . .CH2CH202H+ 02 .O,CH,CH,O,H 10. *02CH2CH202H *CH,CH,OZH + 02 11. * 0 , C H 2 C H 2 0 2 H.HO,CH,CHO,H 1 2 . .02CH,CH20ZH HOzCHCH20,H 1 3 . .OCH,CHO --* CH,O + 'CHO 1 4 . *OCH2CH0 t C?H, HOCH2CHO C2H5. 15. C2H50. CH,0 + CH,' 1 6 . C,H,O. C2H, C?H5OH C,H,'
-
-
-
-+
-
Exptl
Estd
8 .OO
92
9.oo
9.50
03
0 .oo
15l
15.00
29l
29 .OO
8l
8.25
12'
12.00
14'/,'
4 O4
13'
(15.00) 13.70 13.00
215
(42 .OO) 38 .OO 2 1 .oo
134
13.00
135
13.00
13,
(13.60) 12.05 9.40
306
07
(28 .OO) 22.00 0.00
15*
1 5 .OO
2g8
29 .OO
13'
13.00
1 7,
17.00
9' 9
'/,
9%7
Log A (English units)
E (kcal/mol)
Exptl
Reaction
-
Estd
Exptl
Estd
Exptl
914
9.50
10'4
8 .OO
+
1315
1 3 .OO
20'5
20.00
9.50
015
0 .oo
1515
1 5 .OO
1915
19.00
14j5
1 4 .OO
3415
34 .OO
816
8 .OO
1317
12.00
301*
30.00
8l9
8.50
1119
1 2 .oo
9:/,20
9.50
020
0.00
1 7 . CH,. + C,H, CH, + C2H5 18. k H O CO H1 9 . *CHO T 0, CHO,* 20. CHO,? *CHO + 0,
-
21. CHO,. CO, + 'OH 2 2 . C,H,* + C,H,j --+ CjH,* 23. CdH,. CpH, + C?H5* 24. CjH,. + C2H, CJHi, + C2H5' 25. C,H,* + O2 C qH 3 0 2 . 26.C4H,02* -----t C,H,. + 02 27. C,H,O2. c H,cH(cH,),OZH 28. CH,&H(CH,),O2H CAHq-
-
-
-+
14"
14 .OO
35'0
35.00
E (kcal/mol)
-
9%'5
8Y2l6
8.50
15''
15.OO
29"
29.00
1122
11.00
11Z2
11.00
1123
11.00
6!/223
6.50
13''
13.00
02.
21"
1211
12.00
1012
10.00
812
8 .OO
1213
12.00
231J
22 .oo
10'
10.00
8?
8 .OO
22.00
29.CH3CH(CH,),O2H CHjCHCH, + *CH?O?H 30. CH,. + C?H, C,H,. 3 1 . C,H,* -+ C2H,j + CH,. 3 2 . C,H,* + C 2 Hb C7H8 +
2724
26.75
+
-
-
825
8 .OO
1326
1 3 .OO
3426
34 .OO
9 .oo
1127
1 2 .oo
92 7
87225
8.50
C2H5'
--+ A
a Superscripts on the estimates refer to the sources as listed in the Appendix. Experimental values in parentheses refer to the controlled data set when they are different from the uncontrolled data set values. All estimates are rounded values to reflect confidence limits.
olefins and paraffins. This effect is especially predominant at low temperature levels and essentially absent a t high temperature levels. Based upon these experimental results it can be concluded that two processes are occurring during a partial oxidation-oxidation and oxidative-cracking -with oxidation predominant a t low temperature levels and oxidative-cracking a t high temperature levels. The paraffinic products are usually of secondary significance in the partial oxidation systems. The use of a controlled system results in the enhancement of the oxidation processes a t the simultaneous expense of the oxidative-cracking processes and is especially noticeable at the lower temperature levels. Regardless of the type of system utilized, the product distribution becomes quite insensitive to temperature changes a t the high temperature levels. The results of a number of trial and error solutions of the carbon-path mechanism are summarized in Figures 3 162
Ind. Eng.
Chem.. Process Des. Dev., Vol.
14, No. 2,1975
and 4 and Table I, which compares the experimentally derived Arrhenius parameters from both data sets to literature values and correlational estimates. Figures 3 and 4 testify to the excellent correlations which are obtainable with the carbon-path mechanism and Table I indicates virtual correspondence between experimental and estimated Arrhenius parameters which are essentially identical in the case of the controlled reaction system. There appears to be a fundamental reason for the greater consistency exhibited with the controlled data set, and this will form the basis for discussing the particulate solids influence in hydrocarbon partial oxidation systems. At this point, however, the influences of the remaining independent process parameters will be illustrated before the discussions concerning particulate solids are initiated. Pressure Effects. The pressure effects on the ethane partial oxidation system are summarized in Figures 5 and
71
- &
$ y
A
A
2
30--
I
-
IO-
380
400
420
440
460 480 500 Temperature, 'C
520
540
_ -
I
560
580
Figure 1. Summary of the influence of temperature level on the controlled partial oxidation of ethane: pressure, 50 psig; oxidant, air; oxygen to ethane molar ratio in feed, 0.15; residence time, 9 sec. Fused alumina surface. Legend: 0, olefins = A; A , carbon oxides = B; 0 ,oxygenated compounds = C; 0 , paraffins = D.
io- -
380
400
440
420
460 480 500 Temperature, "C
520
540
560
580
Figure 4. Carbon-path mechanism correlation of the partial oxidation of ethane under uncontrolled conditions (conditions as shown in Figure 2).
./
I
I 380
n, 400
420
440
460 480 500 Temperature, "C
~
520
?n! .ri 540
560
,
-1
01
I
50
0
I
580
Figure 2. Summary of the influence of temperature level on the uncontrolled partial oxidation of ethane: pressure, 50 psig; oxidant, air; oxygen to ethane molar ratio in feed, 0.15; residence time, 9 sec. No solids surface.
IO0
,
I
200
I50
Pressure, p s i g
Figure S. Summary of the influence of pressure level on the controlled partial oxidation of ethane at a low temperature level (approximately 400°C): oxidant, air; oxygen t o ethane molar ratio in feed, 0.15; fused alumina surface. 70r--
---
- v - - r
I
i
--
707,
,
~ - - -Ti------
-
k
5
6 0 k
:
i
0
I-
50--
&A
"\
-
-
i
4
-
n
0 0
I
50
-
I
0
I IO0
A
I50
200
Pressure Level, p s i g 380
400
420
440
460 480 500 Temperature, "C
520
540
560
580
Figure 3. Carbon-path mechanism correlation of the partial oxidation of ethane under controlled conditions (conditions as shown in Figure 1)
6 for a low and high temperature level, respectively. Two general features of the ethane partial oxidation system are apparent. First, high pressure levels enhance the formation of low-temperature products to the detriment of hightemperature products; L e . , the oxidation processes are favored over the oxidative-cracking processes at high pressure levels. Second, the influence of pressure variations falls off to essentially zero at pressure levels greater than about 100 psig. The corresponding predictions of the carbon-path mechanism are summarized in Figures 7-10. It is undeniable that there exists a considerable consistency between experiment and prediction. Inspectisn of Figures 5 and 9 or 6 and 10 reveals that despite a prediction of decreasing selectivity to paraffins
Figure 6. Summary of the influence of pressure level on the controlled partial oxidation of ethane at a high temperature level (approximately 550°C): oxidant, air; oxygen t o ethane molar ratio
in feed, 0.15; fused alumina surface. with increasing pressure, the reverse is found to occur experimentally. This discrepancy is not held to be either a fault of the experimental data or the carbon-path mechanism. Rather it is due to a process which by its very nature cannot be quantified as yet. This involves a continued formation of butane from ethyl radicals after all oxygen is consumed. These radicals are generated by the action of chain carriers produced from the remaining secondary initiating hydroperoxides whose selectivity of formation increases with pressure. The product distribution was also predicted a t a very high pressure level (approximately 1000 psig) resulting in very high yields of ethanol (normally a minor product) approaching 90% carbon selectivity. This type of behavior has indeed been demonstrated experimentally by Frolich. Ind. Eng. Chem., Process Des. Dev., Vol. 14, No. 2, 1975
163
70-
I
I
I
I
i
I
I
I
70
\
0
I
i
I
100
50
I
,
I50
0
Figure 7 . Carbon-path mechanism prediction of the influence of pressure level on the controlled partial oxidation of ethane at a low temperature level (approximately 400°C): (conditions as shown in Figure 5 ) .
i
c
100 Pressure L e v e l , psig
0
P r e s s u r e , psig
I50
50
Figure 10. Carbon-path mechanism prediction of the influence of pressure level on the uncontrolled partial oxidation of ethane a t a high temperature level (approximately 560°C): oxidant, air; oxygen to ethane molar ratio in feed, 0.15.
c
0
c
2
20
B
D
0
i
0 3
4
0
0
I
IO0
50
150
0
'
~
"
02
'
~
"
"
"
#
I
01 02 ethane feed
#
I
I 04
Pressure Level, psig
04 05 0 6 0 Oxidant L e v e l , m l e s &/mole
Figure 8. Carbon-path mechanism prediction of the influence of pressure level on the controlled partial oxidation of ethane a t a high temperature level (approximately 560°C): (conditions as shown in Figure 6).
Figure 11. Summary of the influence of oxidant level on the controlled partial oxidation of ethane a t a low and a high pressure level, respectively, with pure oxygen as oxidant: temperature, -400°C: fused alumina surface.
0
01
l
I-
03
Fn
^^
03
-
i'
L01
0
D 1 50
I IO0 Pressure Level, psig
1
I50 Oxidant L e v e l , moles O,/rnole
elhnne feed
Figure 9 . Carbon-path mechanism prediction of the influence of pressure level on the uncontrolled partial oxidation of ethane a t a low temperature level (approximately 400°C): oxidant, air; oxygen to ethane molar ratio in feed, 0.15.
Figure 12. Carbon-path mechanism prediction of the influence of oxidant level on the controlled partial oxidation of ethane a t a low pressure level with pure oxygen as oxidant: temperature, -400°C; fused alumina surface.
Composition Effects. The effects of bulk feed composition upon the ethane partial oxidation system are illustrated in Figure 11 for two pressure levels. There is no indication of any change in bulk feed composition effect with a change in pressure level, and it appears as though the oxidation process is favored a t higher oxidant levels. However, within the oxidation products it is apparent that formation of oxides is enhanced a t the expense of the oxygenated compounds a t the higher oxidant levels. This is not primarily the result of an increased conversion of the formaldehyde product but is caused by the lowered selectivity to stable oxygenated compounds a t high oxygen concentrations. The products of aldehyde reaction are, of course, the oxides. The corresponding predictions of the
carbon-path mechanism for the low pressure level situation is summarized in Figure 12. In correspondence with experiment, the oxys are found to decrease and the oxides to increase within the overall product distribution as the oxidant level within the system is increased. Although the effects of oxidant level in situations in which air is utilized as oxidant are not well documented experimentally, Figure 13 illustrates the predictions of the carbon-path mechanism. There exist only minimal differences in utilizing either air or pure oxygen as the oxidant in that the oxidation processes are less favored over the oxidative cracking processes when air is utilized as the oxidant. This is primarily a result of the diluting effect of the inert nitrogen.
164
Ind. Eng. Chem., Process Des. D e v . , Vol. 14, No. 2,1975
I
t F”C
’
4 0 0 “C ~
-
c 0
01
03 0 4 0 4 06 07 0 01 Oxldant L e v e l , moles O,/mole e t h a n e f e e d
0 2
02
0 3
Figure 13. Carbon-path mechanism prediction of the influence of oxidant level on the uncontrolled partial oxidation of ethane a t a low and high temperature level, respectively, with air as oxidant.
1 5
6
7
8
9
_LL_;j
IO
II
12
Contact Time. s e c
Figure 14. Summary of the influence of contact time on the controlled partial oxidation of ethane: pressure, 50 psig; temperature, -400°C; oxygen to ethane molar ratio in feed, 0.15; fused alumina solids.
Contact Time Effects. Compared to the effects of other process parameters, that of the contact time is of little significance to product distribution in the ethane partial oxidation system as illustrated in Figure 14. Along with a continuing reaction of aldehydes, present in relatively small quantities, to produce oxides, there is also a relatively small additional conversion of feed hydrocarbon occurring a t lower feed hydrocarbon and oxidant concentrations. A slight increase in conversion beyond a certain contact time occurs because of the autocatalytic nature of the conversion process and the limited amount of oxidant within the system. These effects will, according to the carbon-path mechanism, shift the product distribution in favor of the oxides and paraffins at the expense of the oxygenated compounds and olefins. Since oxides and paraffins are normally the least desired products of a partial oxidation and since the conversion of feed hydrocarbon is little affected, it is to the advantage of a partial oxidation process to operate a t the shortest contact times which are practical. A certain minimum contact time must be provided, however, for bulk conversion to occur. Regardless of what the contact time may be, the carbon-path mechanism can accurately account for these processes. Conclusions The carbon-path mechanism has been successfully applied to the correlation and prediction of experimental data. Its usefulness and validity has been demonstrated using only literature and estimated values of Arrhenius parameters. Back calculation of the parameters from the experimental data correspond closely to the assumed values obviating the use of fitting techniques and allowing direct calculation of reaction yields. Thus, the model shown in Table I allows quantitative prediction with conversion as the only necessary input parameter. In addition, the influence of the independent, homogeneous pro-
cess parameters-temperature, pressure, composition, and contact time-can be calculated. The importance of both intramolecular and intermolecular hydrogen abstraction has been established by validation of the mechanism. The most important reactions in the mechanism are the four reactions of the ethylperoxy radical which are necessary for any correlation allowing reasonable predictions. The influences of particulate solids have been alluded to, but have not been discussed in any detail. It is the influences of these non-usual or heterogeneous process parameters, specifically particulate L.?lids, to which attention will be directed in the companion paper. Appendix. A Listing of Some Pertinent Sources of Arrhenius P a r a m e t e r Estimates Referenced in Table I (1) S. W. Benson, “Thermochemical Kinetics,” Wiley, New York, N.Y., 1968. ( 2 ) Polyani Relation for Exothermic Abstraction Reactions. (3) D. P. Dingledy, J. G. Calvert, J. Am. Chem. Soc., 85,856 (1963). -1 (4) C H 3 C H 2 0 2 1 ~CH3CH02H -2 CH3CHO + .OH. Log Al(sec-1) N 14-15 on basis of log ’4 = 12-13 for -CH2CH202H from Benson (S. W . BenCH3CHz02. son, Aduan. Chem. Ser., No. 76, 143 (1968). LogA..l(sec-l) = 14-15 (Isentropic Reaction). E1 2 40 kcal/mol (Ring Strain 26 kcal/mol, endothermicity 8 kcal/mol, normal abstraction 6 kcal/mol). E - 1 e 32 kcal/mol (from endothermicity). LogAz(sec-1) = 13-14. Ez = 10 kcal/mol (maximum). Reaction 2 completely dominates reaction -1, and makes overall reaction irreversible with kinetics of slow step reaction 1. (5) A. Fish, “Rearrangement and Cyclization Reactions of Organic Peroxy Radicals, in Organic Peroxides,” Vol. I, D. Swern, Ed., Wiley, New York, N.Y., 1970. (6) Endothermicity estimated at about 22 kcal/mol from thermochemical data. Typical activation energy of reverse “alkylation” reaction is about 8 kcal/mol; E = 30 kcal/mol. Log A(sec-1) is about 13 based upon analogous reaction CH3CH2CH2 CH3. CzH4 for which Benson (ref 1)estimates LogA(sec-l) 13.6. (7) Estimates are made by analogy to reaction 2 . (8) Estimates are made by analogy to reaction 3. (9) Five-center, secondary hydrogen isomerization step (see ref 5). (10) Four-center, secondary hydrogen isomerization step (see ref 5 ) . (11) Approximated from corresponding values for reaction 15. (12) Approximated from corresponding values for reaction 16. (13) Approximated from referenced values in G. H. Denis, Ph.D. Thesis, The Pennsylvania State University, University Park, Pa., 1973. (14) Approximated from values reported by A. F. Trotman-Dickinson and E . W. R. Steacie, J . Chem. Phys., 19, 329 (1951). (15) Values are only approximate and were not utilized to separate total oxides into carbon monoxide and dioxide. Although parameter estimates indicate dioxide formation may well be by this route, heterogeneous formation to dioxide from monoxide also appears significant. All values approximated from similar reactions and thermochemical data. (16) Approximated by analogy to reaction 30. (17) Approximated by analogy to reaction 31.
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(18) Estimated from endothermicity of the reaction and the activation energy of the reverse reaction. (19) Estimated by analogy to reaction 17. (20) Estimated by analogy to reaction 2. (21) Estimated by analogy to reaction 3. (22) Six-center, secondary hydrogen isomerization step (see ref 5). (23) Estimates from values of the reverse reaction 27 and thermochemical data. (24) Approximated from the analogous reaction 23, and estimated thermochemical data. (25) R. K . Brinton, J . Chem. Phys., 29,781 (1958). (26) Frequency factor taken from estimated value from Benson (ref 1) and the activation energy is estimated from endothermicity and activation energy of reverse, react,ion 30; e.g., E = 25.5 8.5 = 34 kcal/mol, where AH0 = 25.5 kcal/mol and E r e v e r s e = 8.5 kcal/mol. (27) Estimated by analogy to reaction 17.
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Literature Cited Daubert, T E., Jones, J. H., Fenske, M. R.. J. Chem. Eng. Data, 8, 261 (1963). Jones, J. H., Fenske, M. R., lnd. Eng. Chem., 51, 262 (1959).
Jones, J. H., Allendorf, H D , Hutton, D. G., Fenske, M . R., J . Chem. Eng. Data, 6, 620 (1961a). Jones. J. I - . , Fenske, M . R.. Hutton, D. G., Allendorf, H. D., J. Chem. Eng Data, 6, 623 (1961b) Jones, J. H . , Daubert, T. E., Fenske, M . R., lnd. Eng. Chem., Process Des. Develop., 8. 17 (1969a). Jones, J. H.. Daubert, T. E.. Fenske, M. R . . lnd. Eng. Chem., Process Des. Develop., 8, 196 (1969b). Jones, J. W.. Daubert, T. E.. Fenske, M . R., Sandy, C . W.. Lau. P. J,, lnd. Eng. Chem.. Process Des. Develop., 9, 127 (1970) Jones, J. H.. Fenske, M . R., Rusk. R. A . , lnd. Eng. Chem. Prod. Res. Deveiop., 10, 57 (1971a). Jones, J. H., Fenske. M. R., Belfit, R. W., l n d . Eng. Chem., Prod. Res. Develop.. 10,410 (1971b). Svoboda, K . G., Daubert. T. E , lnd. Eng. Chem.. Prod. Res. Deveiop., 11, 337 (1972).
Receiued for reuiea June 26, 1974 Accepted December 9, 1974 Supplementary Material Available. A detailed derivation of the carbon-path mechanism will appear following these pages in the microfilm edition of this volume of the journal. Photocopies of the supplementary material from this paper only or microfiche (105 X 148 mm, 24X reduction, negatives) containing all of the supplementary material for the papers in this issue may be obtained from the Journals Department, American Chemical Society, 1155 16th St., N . W., Washington, D. C . 20036. Remit check or money order for $4.50 for photocopy or $2.50 for microfiche, referring to code number PROC-75-159.
Effects of Forced and Natural Convection during Ultrafiltration of Protein-Saline Solutions and Whole Blood in Thin Channels William J. Huffman,*’ Robert M. Ward,’ and Richard C. Harshman Department of Chemical Engineering, Ciemson Univers/ty, Clemson. South Carolina
The ultrafiltration of 5 % bovine albumin-saline solution and whole blood were studied in a parallel-plate flow cell with one porous wall. Hydrostatic pressure gradients from 55 to 593 m m Hg were applied to obtain 2-85% volumetric separation in channels ranging from 0.020 to 0.165 c m in height. For forced convection conditions, protein-saline solution data were correlated using a modified Leveque-Graetz equation. Increased ultrafiltration rates up to 200% were obtained when natural convection was superimposed on bulk flow. For whole blood, the Leveque-Graetz correlation only applied at low separations. Increased ultrafiltration rates were also observed for whole blood when natural convection flow was permitted.
The extracorporeal removal of toxic substances in blood using a dialysis apparatus or hemodialyzer has been one of the successes of medical science and engineering. Many devices for this treatment have been developed over the years, but widespread use has been restricted because many of the designs have yielded bulky units requiring constant supervision and expensive membrane replacement. More recent improvements using solid adsorbants minimize some of the disadvantages, but the problem with expensive membrane replacement still exists (Hunt, 1973). Further improvements have been proposed through the use of alternate membrane separation processes, and ultrafiltration has been investigated (Bixler, et al., 1968). As is well known, the driving force for the ultrafiltration or reverse osmosis process is the difference between the applied hydrostatic pressure gradient and the osmotic pres-
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Address correspondence to this author at the Department of Chemical Engineering, Texas Tech University, Lubbock, Texas 79409 201efin-Plastics Dept.. Dow Chemical, Freeport, Texas 77541
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sure exerted at the membrane surface by nonpermeating components. Thus, for a given hydrostatic pressure, the rate of ultrafiltration is governed by the removal of stagnant components from the membrane surface (Sourirajan, 1970). In normal engineering practice, this surface concentration polarization may be reduced by mechanical agitation or turbulent flow (Michaels, 1968; Shenvood, 1965; Reilly, 1969), but with fragile materials such as blood these two methods of intense mixing cannot be employed. In addition, the build-up of a gelatinous layer or sedimentation during whole blood ultrafiltration has been reported which further reduces the rate of transfer (Bixler, e t al., 1968) and could limit application due to physiological effects. On this basis, we decided to investigate the contribution of mixing by natural convection to reduce the concentration polarization and minimize deposits or gel layers during ultrafiltration. Experimental Section Chemicals. Bovine albumin was powdered, Cohn Frac-
