Thermal decomposition of propane: An original method of temperature calibration in a plug flow pyrolysis apparatus

15

Journal of Analytical and Applied Pyrolysis, 21 (1991) 15-25 Elsevier Science Publishers B.V., Amsterdam

Thermal decomposition of propane: an original method of temperature calibration in a plug flow pyrolysis apparatus

F. Billaud DPpartement de Chimie Physique des Rkactions, URA No. 328 CNRS, INPL-ENSIC, 54001 Nancy (France)

BP 451,

(Received August 22, 1990; accepted October 9, 1990)

ABSTRACT Accurate temperature measurements are always difficult to obtain in chemical kinetics experiments. It is possible to evaluate the difference between the actual temperature and the measured temperature using calibration techniques. The initial rates of formation of the primary products in the thermal decomposition of propane were measured. As the mechanistic model of the pyrolysis of propane is well known, the corresponding initial rates of product formation were calculated and by comparison the actual temperature in this plug flow reactor was deduced.

Plug flow reactor; propane; pyrolysis; thermal decomposition.

INTRODUCTION

In a tubular electric oven, the temperature is not constant along the axis; it is important to know the temperature profile but, from the kineticist’s point of view, an estimate of the average temperature may be quite useful. Obviously the control thermocouple, located between the reactor wall and the resistors, does not give the actual temperature value. It is possible to estimate the difference between this wall temperature and the actual average temperature in the gas phase when the thermocouple is not located in the gas phase. Accurate temperature measurements are always difficult to obtain in gas phase kinetic experiments, especially in flow systems. In another experimental set-up [l], temperature variations along the tubular reaction vessel of up to approximately 40° C (313 K), depending on the thermocouple position along the thermocouple well, were observed. 0165-2370/91/$03.50

0 1991 Elsevier Science Publishers B.V. All rights reserved

16

Our experimental set up is basically designed to investigate the kinetics and mechanism of the pyrolysis of higher alkanes, cycloalkanes and aromatics, used as model compounds for the representation of petroleum fractions. We performed a preliminary investigation of the thermal decomposition of propane, diluted with nitrogen, between 650 and 750 o C (923 and 1023 IS). This alkane was chosen because a large number of papers have been published on the pyrolysis of propane and therefore its reaction mechanism is rather well known. In a survey on propane pyrolysis published in 1977, Volkan and April [2] mentioned 103 references on this subject. Therefore the reaction mechanism is rather well known and a comparison between the experimental results and the predictions derived from the mechanism is possible.

PYROLYSIS

APPARATUS

The apparatus can be divided into four main sections: injection, preheating, reaction zone, and quench and effluent separation (see Fig. 1). Injection: Nitrogen (N 60 Alphagaz) and propane (N 60 Alphagaz) flows were controlled by two mass flow regulators (RDM Air Liquide) which produce a smooth and time-independent injection. Preheating: The preheating is achieved by means of electrically insulated resistors with an Inconel sheathing (brand name: Thermocoax), directly coiled around the tube (inox 316 L). A control thermocouple is located

mass flow meter

pressure transducer +

C3H8

preheating section

thermocouple and temperature reading

ent

chromatographic sampling loop

Fig. 1. Outline of the pyrolysis

apparatus.

17

between the reactor wall and the resistors; its signal is sent to a temperature controller. We have previously calculated the length of the preheating sections. Reactor tube: The total pressure in the reactor is slightly above atmospheric pressure. Heating of the reactor is obtained by the same device as in the preheating sections. The volume of the reaction zone reactor is V, = 8.6 cm3. Analysis of products: The gaseous reaction products are analysed by gas chromatography. The characteristics of the chromatographic columns and the operating conditions are as follows: Column

Detector

Temperature

Squalane, 10 m, l/4 inch on firebrick (60/80 mesh)

Flame ionisation *

60°C

*

Flame Ionisation Detector (FID) is based on organic compounds they bum in a hydrogen flame, thus causing a measurable current.

EXPERIMENTAL

producing

ions when

RESULTS

The first step in such an experimental investigation is the measurement of the concentrations of the major reaction products (hydrogen, methane,

8-

lo6 x C (mol/l)

T = 923 K (650 “C)

Fig. 2. Pyrolysis

of propane

at 650 o C. Formation

of CH,,

C,H,

and

C3H6.

18

10 5 x C (molll)

T = 973 K (700 “C)

1

‘t

V

8,o

/

,

r

036

094

032

Zcs)

Fig. 3. Pyrolysis of propane at 700°C. Formation

of CH,, C,H,

and C,H,.

ethylene and propene) vs. residence time. From the corresponding curves, it is possible to measure with reasonable accuracy (if the reaction is not too self-inhibited) the initial rates of product formation. The formal relation-

30 1

lo5 x C (mol/l)

T = 1023 K (750 “C)

Fig. 4. Pyrolysis of propane at 750 o C. Formation

of CH,, C,H,

and C,H,.

19 TABLE I Pyrolysis of propane. Experimental determination of the initial rates of products formation (in M L-’ SK’) [(C,H,), is the initial concentration of propane] T=650°C (C,H,),=lS46xlO-3 V’FP(CH4) =1.26x lo-’ M L-’ s-’ ~p(C2H4) =1.2Ox1O-5 ML-’ s-’ VFP(C,H,) =1.59x 1O-5 M L-’ s-’

ML-’

T=7OO”C (C,H,), = 1.468 1O-3 M L-’ VTP(CHq) =1.12x 1O-4 M L-’ s-l Vo’“p(C,H4) =1.27x 1O-4 M L-’ s-l FP(C,H,) =1.20x 1O-4 M L-’ s-’ T= 750°C (C,H,), =1.395 1O-3 M L-’ Vo’“P(CH,) = 4.92x1O-4 M L-’ s-l VFp(C,H4) = 5.5Ox1O-4 M L-’ s-’ VFp(C3Hs) = 5.10~10-~ M L-’ s-l

ships giving these initial rates can also be calculated from the reaction mechanism if the Arrhenius parameters are available. A comparison between the theoretical expression of the initial rates and their experimental values allows the deduction of the actual average temperature inside the reaction tube in which the decomposition reaction takes place. The experimental investigation was performed at three temperatures: 650, 700 and 750°C (923, 973 and 1023 K). For each experimental temperature, the product partial pressures have to be measured at the same partial pressure of propane. For each residence time, since the total pressure PT in the reactor is not constant, it is necessary to adjust the overall flow in order to obtain a constant partial pressure of propane, Pc,H, = xC,_,,PT, where x~,~, is the propane molar fraction. Since it is well known that the concentrations of H, and C,H, are nearly equal [3] and since the measurement of H, is less accurate (detection by thermal conductivity instead of FID), we have only plotted the yields of methane, ethylene and propene vs. time (Figs. 2-4). Using these plots, the least squares method and a model such as C = a72 + by, the tangent to the curve at the origin (b) gives the initial rate for each reaction product. The results as shown in Table 1; T is the experimental temperature, measured using the thermocouple. INTERPRETATION

The theoretical calculation of the initial rate was performed by using the mechanism proposed by Jezequel et al. (3). This mechanism is shown in Table 2; the kinetic parameters A and E of the elementary steps required in the calculation are also given.

20 TABLE 2 Reaction mechanisms of propane pyrolysis; rate constants k = A 10 (- E/4.57 cm3, s) published by Allara and Shaw [4].

--L

‘cy+

.&I-&

84 700 4,57 T

k,=10’4Ssx10

kS=ld”

kc -

Propagation

H.

&H3+

-- 32 000 457 T

x 10

-- 11500 4.57 T

bA=ltixlO

‘GH7@)

T) (units mol,

b=10’38x

10

_- 38000 4.57T

b=lo’=x

10

_- 40300 4.57 T

s-t

s-l

I. mor’d

s-l

c9 I

Propagation

e

k?E

0

’ ‘247

(3)

6” H’ + C&&

I

+

C&+ H.

ka,

km

f%+‘w(s)

Chain crossing

bA= IO@x IO

-- 11500 4.57 T

_lsomerization

k= 10%~x lo

10500 467 T

k&=108X IO

I. mor’.d

-- 17000 4.57 T

I.mor’.d

I. md.d

Ally1 formation

%P = 10’0”

f ‘c&t

*%+

Termination

‘C&

kSB

r&H,

d

iCd-40

*Gb(s)

kps

I

W+

c

_ ’ GH, (9 + ‘GH, (4 L

GH,

W-44 16,

kss L

I.mol~‘.s~’

GHe+ c3HS

=

iO'*'

I.mol~‘.s”

21

It is assumed that the steady state assumption is valid and that the kinetic chains are long. From this radical mechanism and since the Arrhenius parameters are available (4), we can calculate ‘the initial rates of formation of the main primary products (H,, CH,, C,H, and C,H,) as a function of

TABLE 3 Computations of initial rates at 923 K STEP=P,=lOK;

T,=880K;

T,=950K

T (K)

V(1) (M L-l s-l)

V(2) (M L-’

880 890 900 910 920 930 940 950

6.77 9.87 1.41 2.01 2.83 3.95 5.44 7.45

7.07 1.03 1.49 2.14 3.02 4.23 5.86 8.04

STEP=P,=l

K; T,=890K; s-l)

E-06 E-06 E-05 E-05 E-05 E-05 E-05 E-05

s-l)

E-06 E-05 E-05 E-05 E-05 E-05 E-05 E-05

T,=910K T(K)

V(2) (M L-’

890 891 892 893 894 895 896 897

1.03 1.07 1.11 1.15 1.20 1.24 1.29 1.34

T(K)

V(1) (M L-’

890 891 892 893 894 895 896 897

9.87 E-06 1.02 E-05 1.06 E-05 1.10 E-05 1.14 E-05 1.18 E-05 1.22 E-05 1.27 E-05

898

1.32 E-05

899 900 901 902

1.36 1.41 1.47 1.52

E-05 E-05 E-05 E-05

V,,“,(C,H,) = 1.20 x~O-~ ML-’ s-l QCH,) = 1.26 898 x~O-~ ML-’ s-l 899 900 901 902

903 904 905 906 907 908 909 910

1.57 E-05 1.63 E-05 1.69 E-05 1.75 E-05 1.81 E-05 1.88 E-05 1.94 E-05 2.01 E-05

903 904 905 906 907 908 909 910

s-l)

E-05 E-05 E-05 E-05 E-05 E-05 E-05 E-05

1.39 E-05 1.44 1.49 1.55 1.61

E-05 E-05 E-05 E-05

1.67 1.73 1.79 1.85 1.92 1.99 2.06 2.14

E-05 E-05 E-05 E-05 E-05 E-05 E-05 E-05

V(1) = V(CH,); V(2) = V(C,H,); (C3H8),, = 1.546 X 10M3 M L-‘. computation; T,, Final temperature of computation.

K&(C,H,) x10-‘M

= 1.59 L-’ s-l

Tl, Initial temperature of

22 TABLE 4 Computations of initial rates at 973 K STEP = P, = 10 K; T1= 923 K; T2= 1023 K T (K)

V(1) M L-’

923 933 943 953 963 973 983 993 1003 1013 1023

2.91 4.04 5.56 7.58 1.02 1.37 1.83 2.43 3.20 4.20 5.46

STEP=P,=lK;

T,=953K;

T(K) V, (M L-’

s-‘)

953 954 955 956 957 958 959 960 961 962 963 964 965 966 967 968 969 970 971 972 973

7.58 E-05 7.82 E-05 8.06 E-05 8.31 E-05 8.57 E-05 8.83 E-05 9.10 E-05 9.38 E-05 9.66 E-05 9.96 E-05 1.02 E-04 1.05 E-04 1.08 E-04 1.12 E-04 1.15 E-04 1.19 E-04 1.22 E-04 1.26 E-04 1.30 E-04 1.33 E-04 1.37 E-04

s-l

V(2) M L-’

E-05 E-05 E-05 E-05 E-04 E-04 E-04 E-04 E-04 E-04 E-04

s-l

3.11 E-05 4.34 E-05 5.99 E-05 8.20 E-05 1.11 E-04 1.50 E-04 2.01 E-04 2.67 E-04 3.53 E-04 4.63B04 6.05 E-04

T,=973K T(K)

953 954 955 956 957 958 959 960 961 962 963 964 965 V,,O,(CH,) = 1.12 966 x~O-~ ML-’ s-l 967 968 969 970 V’&,H,) = 1.27 971 x 1O-4 M L-’ s-l 972 973

V(l) = V(CH,); V(2) = V(C,H,); ture of computation.

V(2) (M L-’ 8.20 8.46 8.72 9.00 9.28 9.57 9.86 1.01 1.04 1.08 1.11 1.14 1.18 1.12 1.25 1.29 1.33 1.37 1.41 1.45 1.50

E-05 E-05 E-05 E-05 E-05 E-05 E-05 E-04 E-04 E-04 E-04 E-04 E-04 E-04 E-04 E-04 E-04 E-04 E-04 E-04 E-04

s-l)

V&(C,H,) = 1.20 x~O-~ M L-’ s-l

T,,Initial temperature of computation; T,,Final tempera-

23

the initial concentration obtain:

of propane (C,H8)o at the inlet of the reactor. We

V” CH, = V&j, =

TABLE 5 Computations of initial rates at 1023 K STEP=P,=lOK;

T,=lOOOK;

T,=1033K

i’- (K)

V(1) (M L-’

1000 1010 1020 1030

2.74 3.59 4.68 6.07

s-l)

I’(2) (M L-’

E-04 E-04 E-04 E-04

3.01 3.96 5.18 6.74

s-l)

E-04 E-04 E-04 E-04

STEP = Pz = 1 K; T, = 1010 K; T2 = 1030 K T(K)

V(1) (M L-’

1010 1011 1012 1013 1014 1015 1016 1017 1018 1019 1020 1021 1022 1023 1024 1025 1026 1027 1028 1029 1030

3.59 3.69 3.79 3.89 4.00 4.10 4.21 4.33 4.44 4.58 4.68 4.81 4.93 5.06 5.20 5.34 5.48 5.62 5.77 5.92 6.07

E-04 E-04 E-04 E-04 E-04 E-04 E-04 E-04 E-04 E-04 E-04 E-04 E-04 E-04 E-04 E-04 E-04 E-04 E-04 E-04 E-04

s-l)

T(K) 1010 1011 1012 1013 1014 1015 1016 1017 1018 1019 1020 1021 V-JCH,) = 4.92 1022 x 1O-4 M L-’ s-l 1023 1024 1025 <;,,(C,H,) = 5.50 1026 x~O-~ M L-’ s-l 1027 1028 1029 1030

V(1) = V(CH,); V(2) = V(C,H,); ture of computation.

V(2) M L-’ 3.96 4.07 4.18 4.29 4.41 4.53 4.66 4.78 4.91 5.04 5.18 5.32 5.46 5.61 5.76 5.91 6.07 6.23 6.39 6.56 6.74

E-04 E-04 E-04 E-04 E-04 E-04 E-04 E-04 E-04 E-04 E-04 E-04 E-04 E-04 E-04 E-04 E-04 E-04 E-04 E-04 E-04

s-l

V,,O(C,H,) = 5.10 x~O-~ M L-’ s-’

T,, Initial temperature of computation;

T2, Final tempera-

24

V/" CA

=

vH”,=

k with

'0'

k,,A(l

(l+y)k-,(C,H,),+cw(k,A+k,,)

+;;(C,H&

and

Ro= (1 +dk&3W,+k2,

k,Tl andyzk=-=-= x’

k,rt,

k,rl,

k 3'A

kYB

1

a =

0-b) (C,H,)

=l

A computer code with a temperature increment per step integration of 10” C (283 K), limited to 1” C (274 K) near the experimental value, allows computation with the above expressions, the theoretical values /of Vc’& G& I&l, and VG2 which are listed in Tables 3-5. In these tables, we compare Vc&,, VczH4, Vc:H, with the corresponding experimental initial rates measured in our experiments. At 650, 700 and 750 o C (923, 973 and 1023 K) (experimental temperature: ET), the comparisons give the following values; the arithmetical average temperature (AT) is also calculated. ET

V,(CK,) * ?dCH,) KdC2fLd

-

7'dC,W

G',(C,W

-

7xC3%)

AT

650°C (923 K)

700°C (973 K)

750°C (1023 K)

897 896 902 898

966 971 966 968

1022 1026 1020 1023

K K K K

K K K K

K K K K

DISCUSSION AND CONCLUSION

The previous results suggest that in our experimental set-up, the temperature readings are in good agreement with the computed values, at least at sufficiently high temperature (700 to 750” C) (973 to 1023 K). At lower temperatures (650°C) (923 K) and at lower conversion of propane the agreement is less satisfactory. It is true that it is difficult to understand why at the lowest temperatures the experiment is the least satisfactory by reason of too low a conversion of propane. Theoretically, an extrapolation to zero conversion should lead to the best agreement when the conversion is low. In our case at 923 K, the less satisfactory agreement is not due to a fundamental problem but to the fact that at low conversion of propane, the product formation is low and therefore it becomes more difficult to determine the initial rates of product formation without subsequent error.

25

This technique of temperature apparatus.

calibration

can be used in any pyrolysis

REFERENCES 1 2 3 4

F. Billaud and E. Freund, Ind. Eng. Chem. Fundam., 25 (1986) 433. A.G. Volkan and G.C. April, Ind. Eng. Chem. Process Des. Develop., 16 (1976) 429. J.Y. Jezequel, F. Baronnet and M. Niclause, J. Chim. Phys., 80 (1983) 455. D.L. Allara and R. Shaw, J. Phys. Chem. Ref. Data, 9 (1980) 523.