Partial Oxidation of Light Hydrocarbons. 3. Mechanism Incorporating Key Surface Reactions with Gas-Phase Steps Suresh Mahajan and Lyle F. Albright" School of Chemical Engineering, Purdue University, West Lafayette, lndiana 47907
A mechanism is proposed that explains all major differences of the experimental results for partial oxidation of light paraffins in various reactors. Differences explained include the kinetics and products of partial oxidation. The mechanism includes several surface reactions that have not been considered in previous mechanisms, but these reactions have recently been found to be of importance.
Mahajan et al. (1977a,b) have presented considerable information concerning key reactions and other phenomena occurring on the inner walls of reactors used for partial oxidations. These surface reactions plus the gas-phase reactions reported earlier (Semenov, 1958, 1959; Shtern, 1964; Knox, 1968; Fish, 1968; Euker and Leinroth, 1970) have now been combined for the mechanism for partial oxidation.
Proposed Mechanism Table I summarizes how surface reactions have such a large effect in many cases on the kinetics of oxidation and on the products obtained during partial oxidation of light hydrocarbons. Reactions A, B, C, and D shown in the upper portion of Table I are gas-phase reactions. When the ratio of paraffin to oxygen in the feedstream is large, reaction A is the most important reaction and oxygenated compounds (alcohols, aldehydes, ketones, etc.) are major products for gas-phase reactions. Reactions E, F, G, and H shown in the lower portion of Table I occur on the surface of the reactor. Carbon dioxide, water, and to a lesser extent carbon monoxide are the major products for the surface reactions. It should be emphasized that reactions A through H are each relatively complicated, and to some extent they are overlapping and even incomplete. For example, acetaldehyde may be oxidized in the gas phase to produce carbon monoxide and carbon dioxide; such oxidations would be placed in categories B and C. Yet when acetaldehyde is oxidized, other oxygenated hydrocarbons (formaldehyde, methanol, and acetone) are also produced; production of other oxygenated products as a result of acetaldehyde reactions are not shown in Table I. The following comments are pertinent relative to Table I. (a) Reactions A, B, and C are free-radical reactions that can be divided into initiation, simple chain sequences, and chain-branching sequences that result in an increased concentration of free radicals, and finally termination steps (Semenov, 1958, 1959; Shtern, 1964; Knox, 1968). There is as yet no complete agreement on the specific details of the complicated series of consecutive and simultaneous gas-phase reactions because in part some surface reactions always occur. (b) Reaction D is probably of little importance. A mixture of carbon monoxide and oxygen is almost nonreactive when a Pyrex reactor is used a t temperatures and pressures normally used for partial oxidation runs (Mahajan, 1972). In addition, when tagged carbon monoxide was added to a feed stream containing a paraffin and oxygen, relatively little reacted (Minkoff and Tipper, 1962). (c) Reaction E is probably also relatively unimportant. There is no direct evidence that oxygenated products are
produced on the surface from paraffins or olefins. Euker and Leinroth (1970) indicate, however, that olefins are formed on the surface from paraffins. If oxygenated products were formed in the surface, probably most of them would react by means of oxidation or decomposition steps before they would diffuse into the gas phase. The possibility that trace amounts of oxygenated compounds do form on the surface must, however, be retained. Mahajan et al. (197713) found, for example, that dimethyl ether was apparently formed on t,he surface in reactions involving methanol; they had used conditions comparable to those used during partial oxidation. Dimethyl ether is generally not detected though during partial oxidations even when methanol is a major product. (d) Reactions F occur to at least some extent in all reactors including glass reactors. Both surface oxidation or surface decomposition reactions are included in this category. These reactions are of major importance as will be discussed later in affecting the kinetics and composition of the final product for partial oxidation. (e) Reactions G and H, both surface oxidations, are often of importance in copper and steel reactors, but are a t most only a relatively minor importance in glass or aluminum reactors. In the copper and steel reactors, metal oxides act as oxidation catalysts. Metals or metal oxides, including alumina, also act catalytically promoting decomposition of oxygenated compounds.
Discussion Proposed Model Major differences in both the kinetics and the composition of the product obtained during partial oxidation runs in various reactors depend primarily on the relative importance of surface reactions and particularly reactions F. Destruction of oxygenated compounds directly affects the composition of the product formed, but even more importantly it acts to suppress the formation of additional oxygenated compounds. The major importance of aldehydes is due to the chain-branching sequences into which they enter. A sequence that involves acetaldehyde, oxygen, and propane is shown below.
+0 2 CH3CO. + 0 2 CH3(CO)OO*+ C3H8 CH3CHO
+
+
---*
CH3(CO)OOH
CH3CO-
+
+H0y
CH3(CO)OO*
CH3(CO)OOH
CH3-
+ C3H7-
+ CO2 + HO.
(1)
(2) (3)
(4)
In the above sequence, there is a net increase of four free radicals; such an increase results in increased rates of oxidation as oxygen conversions increase in the range of relatively low conversions. This and similar chain-branching sequences, as has also been explained by earlier investigators, results in Ind. Eng. Chem., Process Des. Dev., Vol. 16,No. 3, 1977
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Table I. Summary of Gas-Phase and Surface Reactions Occurring during Partial Oxidation of Paraffins B
A I
I I
I
-
D ~
I i i 1 i
Paraffiy
II
C
Oxygenated
t
L----------A
~
t Carbon
dxre
Carbon Monoxide
I
t I
t
I
G
A, B, C, and D are gas-phase reactions E, F, G, and H are surface reactions Relative importance of surface reactions varies in reactors as follows:
copper > steel > aluminum > Pyrex old (or used) > new (or unused)
maximum rates of oxidation a t intermediate oxygen conversions, i.e., type I kinetics as reported by Mahajan e t al. (1977a). Another key consequence of chain-branching sequences such as shown above is the increased production of alkyl radicals that are readily converted by gas-phase reactions into oxygenated compounds. In the above sequence, methyl and propyl radicals were formed. Furthermore, both hydroxy (HO.) and hydroperoxy (HOz-) radicals also react, resulting in increased amounts of alkyl radicals and in increased rates of reaction. The chain-branching sequences also result in increased amounts of oxygenated products. When, however, reactions F are important, fewer oxygenated products will be formed and the degree of combustion (Mahajan et al., 1977a) will be higher (Le., more carbon dioxide and water). Propane is shown in reaction 3 as the hydrocarbon that supplied the hydrogen atom that was transferred. Other hydrocarbons also permit hydrogen transfer. Chou (1975) has presented information on the chain-branching sequences of formaldehyde during the partial oxidation of methane. Alkyl hydroperoxides that can be formed from alkyl radicals also are chainbranching materials. Chou’s results indicate that they are significantly less important than aldehydes, however, for chain-branching. The mechanism proposed in Table I explains all experimental results discussed below. (a) As reported by many investigators, the materials of construction used for the reactors have a major effect on the relative importance of surface reactions especially a t lower pressures, e.g., 1atm. The levels of surface reactions (reactions F, G, and H) were demonstrated by Mahajan et al. (1977b) to be as follows: copper > steel > aluminum > glass. Partial oxidation results in tubular reactors of the above materials of construction (Mahajan et al., 1977a) are consistent with the above ordering of the materials of construction. Partial oxidation reactions with type I1 kinetics and with high degrees of combustion occurred when steel and especially copper reactors were operated a t atmospheric pressure. Type I kinetics and relatively low degrees of combustion were noted in glass reactors a t comparable pressures. I t should be emphasized that both surface oxidation and surface decomposition reactions occur to a considerable extent in copper and steel reactors. Aluminum is relatively effective for catalyzing the decomposition of oxygenated compounds, but is not for surface oxidations. Glass, however, generally results in only a small amount of surface decompositions un280
Table 11. Consecutive and Simultaneous Reactions Occurring in the Gas Phase and on the Surface of the Reactor during Partial Oxidation of Light Paraffins Gas Phase Reactions oxygenated compounds, CO, C o n- , HzO Paraffin + oxygen ._ olefins, methane, Hz, etc. Surface Reactions Oxidation of inner surfaces with oxygen to form oxidized surfaces (often metal oxides). Reduction of oxidized surfaces (often metal oxides) with oxygenated compounds, paraffins, olefins, CO, and hydrogen to produce COz, CO, and HzO. Decomposition of oxygenated products on surfaces to yield CO, Hz, coke (or carbonaceous products) and by-products. Oxidation of coke (or carbonaceous deposits) with oxygen to produce primarily COz; CO, HzO, and Hz may be produced in small amounts.
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less, as will be discussed later, there is a high surface-to-volume ratio in the reactor. (b) As metal reactors are used, the surfaces tend to roughen because of repeated oxidation-reduction sequences that occur on the surface. With rougher surfaces, surface reactions (reactions F, G, and H) increase in relative importance with the resulting expected changes in the nature of the kinetics and of the composition of the product. (c) As glass and aluminum reactors are used for partial oxidation experiments, the kinetics of oxidation and generally to a lesser extent the product composition often change. Surface oxidations are of relatively little importance in these reactors as might be expected since both silica and alumina are stable a t oxidation conditions. Carbonaceous deposits do form, however, on the surface during partial oxidation. These deposits cover the silica and alumina surfaces that act as catalysts for the decomposition of oxygenated products (Mahajan et al., 1977b). The higher rates of oxidation as noted in used reactors are hence consistent with such an explanation. Some roughening of the surface does occur slowly as both glass and aluminum reactors are used for partial oxidations. Such roughening would increase the relative importance of surface reactions, as already discussed. (d) Increased operating pressures for partial oxidation experiments reduce the ratio of surface area to mass of reactants; the result is a net decrease in the importance of surface reactions. The results of Mahajan et al. (1977a) and of Kao (1969) show the expected changes of both the kinetics of oxidation and the type of product in the pressure range of 1-4 atm. Using either a steel or copper reactor a t 1 atm. pressure, type I1 kinetics and high degrees of combustion result. At 4 atm pressure, however, type I kinetics and low degrees of combustion occurred. At 4 atm, fairly similar results were obtained for comparable runs in metal and glass reactors. Increased adsorption of water on the surface of the reactor may also be a factor a t higher pressures for minimizing the relative importance of surface reactions. The adsorbed water probably reduces the catalytic or oxidative abilities of the surface. (e) The surface-to-volume ratio of the reactor has a major effect on both kinetics and product composition a t high ratios. Two methods have been used to increase this ratio in glass reactors. First, reactors have been packed with glass wool or glass beads. Second, tubular reactors with small diameters have been used. In both cases, the kinetics of oxidation were drastically reduced (Pease, 1929; Norrish and Reagh, 1940; Chou, 1975), and essentially no oxygenated products were formed. Any oxygenated products formed were apparently destroyed in such reactors primarily on the glass surfaces, and
few if any chain-branching reactions occurred. Carbon oxides and water were always the main products. Table I1 indicates how the many surface and gas-phase reactions are both consecutive and simultaneous. Oxidation and reduction of the surface is a sequence repeated many times until a more or less dynamic equilibrium is approached. Complete equilibrium is, however, never realized since the oxidation-reduction sequence results in a continually increasing roughness of the surface. The level of oxidation of the surface will depend on the material of construction, the roughness of surface, and the axial position in the tubular reactor. Simultaneous production and burnoff of coke or carbonaceous deposits also occurs. Near the exit end of a tubular reactor, the net rate of deposits produced will be greatest since the amounts of oxygenated hydrocarbons are highest and the amount of oxygen is lowest. Experimental information confirms that coking is greatest near the tube outlet. Start-up phenomena noted in many reactors can be explained by means of Table 11. More or less dynamic equilibrium conditions are aproached during start-up relative to the levels of surface oxidation (or surface oxides) and the coke present. The importance and even type of surface reactions often varies significantly during such start-up; differences in both the kinetics of oxidations and the products obtained can hence be explained. The surface reactions described in Tables I and I1 occur at similar operating conditions including temperature regardless of the paraffin being oxidized. Yet gas-phase reactions for the paraffins may require quite different operating conditions, as previously noted (Mahajan et al., 1977a). Steam may sometimes also react at the surface, particularly a t temperatures of 500 "C or above. Mahajan (1972) found that steam reacted to a minor extent at the surface of a steel reactor to produce metal oxides and hydrogen. Steam in the presence of a suitable catalyst also results in shift reactions. The following reaction was also detected in metal reactors.
2co c + coz +
Destruction and initiation of free radicals also certainly occurs on the surface. Although the mechanism proposed here is thought to be applicable for all paraffins, certain features still need clarification. More information is needed on the various interactions between the many gas-phase and surface reactions. One phenomenon that has not yet been explained is the deacti-
vation of glass or metal reactors when they are contacted with acetaldehyde (Albright and Winter, 1966; Holtzmeier and Albright, 1969). Yet the same reactors are often activated as they are used for partial oxidation investigations. In both cases, coke (or carbonaceous deposits) is formed; such deposits presumably cover some catalytic sites that destroy aldehydes. It is of course realized that the amounts and possibly types of coke or surface oxides are probably considerably different in the same reactor after these two types of treatments. In any case for acetaldehyde-treated reactors, higher temperatures were required to obtain significant partial oxidation reactions. Since the composition of the product stream did not change appreciably because of the acetaldehyde treatment, reactions promoting destruction or initiation of free radicals may be important in such cases. In conclusion, more information is still needed concerning the exact compositions of coke and surface oxides on the surface in different reactors, at various axial positions, and under a wide variety of operating conditions.
Literature Cited Albright. L. F., Winter, E. M., lnd. Eng. Chem., Prod. Res. Dev., 5 , 244
(1966). Chou, T. C., Ph.D. Thesis, Purdue University, 1975. Euker, C. A,, Leinroth, J. P., Combust. flame, 15,275 (1970). Fish, A,, Angew. Chem., Int. Ed. Engl., 7 (l),45 (1968). Holtzmeier, L. R . , Albright, L. F., "Twelfth Symposium (International) on Combustion", pp 375-383,Combustion Inst., Pittsburgh, Pa., 1969. Kao, Che-I., Ph.D. Thesis, Purdue University, 1969. Knox, J. H., Adv. Chem. Ser., No. 76, 1 (1 968). Mahajan. S..Ph.D. Thesis, Purdue University, 1972. Mahajan, S., Menzies, W. R., Albright, L. F., lnd. Eng. Chem.,Process Des. Dev.,
16,271 (1977a). Mahajan, S..Nicholas, D. M., Sherwood, F., Menzies, W. R., Aibright, L. F., Ind. Eng. Chem., Process Des. Dev., 16,275 (1977b). Minkoff, G. J., Tipper, C. F. H., "Chemistry of Combustion Reactions", Butterworths, London, 1962. Norrish, R. G. W., Reagh. J. D., Proc. Roy SOC.London, Ser. A, 176, 429
(1940). Pease, R . N.. J. Am. Chem. Soc., 5 1 , 1839 (1929). Semenov, N. N., "Some Problems in Chemical Kinetics and Reactivity', (English Translation), Vol. I. pp 101-1 14;Vol. (I, pp 43,217,Princeton University Press, Princeton, N.J.. 1958,1959. Shtern, V. Ya., "The Gas Phase Oxidation of Hydrocarbons", McMiilan. New York, N.Y., 1964.
Received jor recieu: July 7, 1975 Accepted February 9,1977 Financial support for this project was provided by grants from National Science Foundation, Indiana Gas Association, and the US. Public Health Service. The three papers published here were presented a t the 168th National Meeting of the American Chemical Society, Atlantic City, N.J., Sept. 8-13, 1974.
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