Kinetics of the Fischer—Tropsch Synthesis on Iron Catalysts. I. Rate Data on Reduced and Nitrided Catalysts

Abstract

Rate Data on Reduced and Nitrided Fischer-Tropsch Catalysts 1039 larger diameter range of 80-200 Á. increases regularly with beating time.Despite the theoretical uncertainties in Kelvin equation applications such as the Pierce calculation, the conclusion is inescapable that the ball-mill beating of wood cellulose creates additional pores or fissures in the amorphous regions of the fiber, "or at least enlarges existing pores, all the way down to 50 A. or so in pore diameter. Relation to Fine Structure of Cellulose.-The break in the descending isotherm between relative pressure 0.50 and 0.45 (Fig. 1) appears consistently in all cellulose samples, beaten and unbeaten, and has been independently observed by four different experimenters in these laboratories, as well as by Hunt, Blaine and Rowan.aa Even though any present method of calculating the absolute value of the pore sizes and pore volumes concerned may be open to some criticism as to accuracy, the generalized conclusion from these facts is inescapable, viz., wood and cotton cellulose in the native, swollen state contain a sizable volume (ca. 0.05 cc./g.) of pores rather narrowly distributed in a range with a median pore diameter of about 40 A. The results of this particular investigation further indicate that the volume of these very small pores does not change appreciably with moderate amounts of wet maceration but does increase somewhat with severe and continued mechanical treatment in the wet state.The inference from these observations and conclusions is that the 40 A. diameter pore size is a characteristic of the fine structure of wood cellulose fibers and may be related to the basic structural unit in wood cellulose. It may be hypothesized, for example, that the elementary building blocks of "solid" cellulose are laid down in some sort of array (e.g., hexagonal) permitting the regular occurrencejof "holes" approximately the size of an elementary polymer unit. It is not difficult to imagine several possible combinations of structural units, consistent with the known Xray identity periods of cellulose, which would permit such regular occurrence of holes. The elucidation of such possibilities should provide a fertile field for further research. ConclusionsThe increase in total (internal and external) surface of wood cellulose fibers upon wet maceration in a ball mill is a linear function of time and is principally accounted for by the increase ip volume of submicroscopic pores in the range of 44-800 A. diameter. The most common pore size of 38 Á.diameter is unaffected by beating and the total volume in the range about that pore size is but slightly affected. It is suggested that this most common pore diameter is a fundamental property of the cellulose and that it is structurally related to the elementary polymer unit of cellulose.

Graphical Differentiation in Ammonia Synthesis Kinetics

“…Log-log plots of kinetic data for the Fischer-Tropsch synthesis on nitrided iron catalysts are approximately linear (7). Smoothed data were differentiated by the present graphical method and by a numerical central difference method involving fitting a four-constant poly-nominal to the reference point and three equal intervals above and below it by least squares.…”

“…(1 /S'), where = 1,2, or 3, a is the mole fraction of ammonia, and S is the inlet space velocity (2,4,8,9). Recent workers (9) have used n -2, which is identical to Equation 1, and this equation is obtained by both contact-time and flow-balance derivations (7). The three definitions lead to only small numerical differences, as the term (1 + a) usually has a value between 1.00 and 1.10.…”

“…Equation 5 may be rearranged to {KPHnW/í?) ~1¡ h = (l/-i2)(P/z3A2)l -<* (7) where h, n, and a are mole fractions of hydrogen, nitrogen, and ammonia, respectively.…”

Measuring Thermal Effects in Catalytic Reactions

“…In kinetic studies on fixed beds of catalyst D3001 reported from this laboratory (7,9, 70), the temperatures were lower and the amount of synthesis gas reacted per unit time-weight of catalyst was smaller. Temperatures measured within the bed were usually less than 1°C.…”

“…Equation 1 has been shown to represent data satisfactorily for the Fischer-Tropsch synthesis in packed beds, at least for 0 < x < 0.6, and temperatures below' 270°C. (9). According to Equation 1, the differential reaction rate, r, is given by…”