Determination of Some Important Parameters

Determination of the Arrhenius Parameters The Arrhenius parameters (A and E) for the thermal decomposition of the sample [52-54] were determined assuming a first-order chemical reaction (n = 1). The rate constant is defined as:

where A = preexponential factor, min-1 E = activation energy, kJ/mol K R = universal gas constant, J/mol K T = temperature, K t = time, s

This equation defines the temperature dependence of the specific rate constant. A second equation that relates the reaction progress to time through the rate constant is also required:

Integrating yields the following:

where a = reaction progress

In the last equation, the right-hand side has no exact solution, and several approximate solutions to this equation have been used to determine the order of reaction, activation energy, and preexponential factor:

Taking the natural logarithm of this equation yields:

The value of E and ln(A) was evaluated from the slope and intercept of function ln[- ln(1 — a)] versus 1/T [49, 52]. The data to construct this plot are taken from the TGA curve; see Figure 15.23.

Determination of the Maximum Decomposition Temperature The value of maximum decomposition temperature was calculated from the following two equations:

Figure 15.23. 1/T vs. ln[ln(1 — a)] for determining Arrhenius parameters.

Notice that in the second of these two equations, the point of maximum decomposition can be obtained by plotting dm/m versus time or temperature, and is the minimum of this curve where mi = initial mass.

Determination of the Heat of Pyrolysis The value of the heat of pyrolysis was calculated from the following two equations:

1 ft2

where h = heat of pyrolysis mi = initial mass P(t) = heat flow t = time

Estimation of Required Heat of Pyrolysis for Different Compounds Polyethylene (PE) is usually referred as (C2H4)n, or h2

The total molecular weight of polyethylene is:

Mw = 200,000 - 300,000 Polypropylene (PP) is usually referred as C3H6, or

H2 H

Polystyrene (PS) is CgHg

¿PE,PP,PS = Na/Mw X Mw/Mw(unit) X AHic/Na = AHic/Mw(unit)

Na/Mw X Mw/Mw(unit) X H(c-C)/Na = H(C-C)/M w(unit) Polyvinyl chloride (PVC) is C2H3CI, or h h

—^ Sc^n* + hcl rc n* h cl hPvc = [(H(c-ci) + H(c-h) + H(c-c)) — (H(h—el) + H(c=c))]/Mw(h—ci) Cellulose is C6H10O5, or ch2oh ch2oh

Was this article helpful?

0 0
Building Your Own Greenhouse

Building Your Own Greenhouse

You Might Just End Up Spending More Time In Planning Your Greenhouse Than Your Home Don’t Blame Us If Your Wife Gets Mad. Don't Be A Conventional Greenhouse Dreamer! Come Out Of The Mould, Build Your Own And Let Your Greenhouse Give A Better Yield Than Any Other In Town! Discover How You Can Start Your Own Greenhouse With Healthier Plants… Anytime Of The Year!

Get My Free Ebook


Post a comment