Surface Effects in Butane Oxidation

Abstract

T h e oxidation o f butane is considerably affected b y the nature o f the vessel surface. W i t h a silica vessel, vigorous cornbustion acconlpanied b y multiple cool flarnes occurs between INTRODUCTIONThere is considerable evidence that the oxidation of hydrocarbons is subject to surface effects. ICinetic experimeilts have revealed a dependence of rate on vessel diameter (1, 2) and on the previous treatment of the vessel walls (3, 4, 5). Although systematic investigations of surface effects have been made with hydrogen (6, 7, 8) ancl methane (4, 9) there is a lack of information about the role of the surface in the oxidatioil of the higher hydrocarbons, where mechanisms not foui~cl with inethaile come into play (10, 11). A better understanding of the part played by the surface in these reactions is desirable for several reasons: it would clarify some of the puzzling anoinalies in the reaction lcinetics (12, 13); it might suggest improvements in petrochemical processes; finally, it might help reveal the mechanism of "ltnoclt" and antiknock action in engines.Although l c n~~l e and its suppression have been subjected to much investigation (14, 15, 16) there is still considerable disagreement about the nature of the process. T h e work of Icing and co-workers (17, 18, 19) and of Walsh and co-worlcers (20, 21, 22) inay be cited as illustrating both the lack of agreement 011 mechanism and the possible iinportance of heterogeneous processes.ICing (17, 18) ascribes the action of antiknoclc agents such as iron carbony1 or lead tetraethyl to depositioil of metals or metal oxides ~vhich are believed to proillote the formation of steam and oxides of carbon; these gases inay then dilute the con~bustible mixture and reduce its inflaminability. 21,22) ascribe the antiltnock action of lead tetraethyl to the formation of a colloidal smolce of lead monoxide, which is assumed to inhibit the lcnoclciilg reaction by virtue of a surface destructioil of free radical chains. Each group of investigators claims supporting evidence from combustion experiments. The positive-catalyst theory favored by King is said to be supported by the results of flow experiments with fuels such as pentane and hyclrogen (18, 19). The negative-catalyst theory favored by Walsh finds support froin static experimellts wit11 fuels such as methane and ethers (21,22).The question of antiknock action has continued to be a contentious one (IG,23,24). I t appeared desirable to establish empirically the effects of various inorganic compounds on the coinbustion behavior of a typical hydrocarbon. For this investigation the fuel chose11 was n-butane, which oxidizes a t collvenient rates between 250' C and 500' C. This compound's oxidation is accompanied by cool flames, two-stage ignition, and other phenomena associated with the lorn-temperature combustion of paraffin hj7drocarboils and related substances (11,25).

A numerical study of cool flame development under microgravity

“…However, if the boundary conditions are transformed to permit full destruction of B at the surface, as in equation 6, this so diminishes the reactivity that strong exothermic reaction becomes possible in reasonable timescales only at T a Ͼ 650 K, where highly damped oscillations are inevitable. This is consistent with the experimentally observed suppression of cool flames when efficient chain terminating surfaces are used [19]. There was also no indication of a propagating front when these boundary conditions were applied.…”

A Tentative Modeling Study of the Effect of Wall Reactions on Oxidation Phenomena

“…The introduction of the wall reactions with unadjusted rate coefficients raises the ignition boundary from a temperature of 500 K to a position at 660 K. Figure 6 shows the result of dividing the rate constants for the wall reaction (W4) by a factor of 2 or 10. Dividing by 2 results in a decrease in the position of the autoignition boundary by 25 K, whereas a division by 10 decreases the value more strongly to 590 K, bringing it close to the experimental value 3 for an NiO wall coated vessel, shown in figure 1. effect.…”

“…Turbulent convection may occur in larger vessels and at higher pressures. [7][8][9] At differing experimental conditions (T a = 557 K and p = 32 kPa) two consecutive cool flames occur with an induction time of the first one of 90 s. 3 In this case τ reaction /τ convection ~ 24.3, so a greater extent of mixing will have been achieved compared to the ignition case. With such order of magnitude values for the diffusion and convection timescales it is difficult to say categorically that the assumption of a uniform gas mixture in the untreated vessel is perfectly valid well before the onset of the cool flame, but it is likely that a good degree of mixing has taken place and this will increase as the temperature further rises.…”

“…The effects of surface treatment on the internal wall of a 254 cm 3 cylindrical reaction vessel have been studied by Cherneskey and Bardwell. 3 The p-T a ignition diagrams for equimolar n-butane/oxygen in an untreated silica vessel, a vessel internally coated with NiO and a vessel internally coated with PbO, as shown in figure 1, reveal that ignition and cool flame boundaries are modified by the introduction of such coatings, with the greater inhibitory effects being observed with PbO. A marked increase in the pressures and temperatures required to bring about autoignition are observed for the NiO coated vessel with further increases for the vessel coated with PbO.…”

Detailed chemical kinetic models for the low-temperature combustion of hydrocarbons with application to gasoline and diesel fuel surrogates

“…As reviewed by Pollard [47] and Griffiths [23], closed vessels have been much used before 1993 to study the low temperature oxidation of hydrocarbons and other gaseous organic compounds. Measurements of autoignition delay times in such an apparatus can be strongly sensitive to wall reactions, as shown for n-butane by Cherneskey and Bardwell in coated reactors [147].…”