30 Ind. Eng. Chem. Process Des. Dev. 1985, 24, 30-37

Acknowledgment The authors wish to express their appreciation to Chi-

yoda Chemical Engineering and Construction Company for supporting the present study and for permitting the publication of this paper. Nomenclature b, = upper bound off value imposed in order that the se-

quence order of P, holds at the nth array defining feasible regions of solution

C, = specific heat of flue gas fi = fraction of ith heat demand between ith and i - l th heat

GF = flow rate of flue gas hBFW = specific enthalpy of boiler feed water hi = specific enthalpy of steam supplied for ith heat demand h$ = specific enthalpy of steam used up for ith heat demand h, = specific enthalpy of high pressure steam generated at

H . = steam demand at the jth steam level H', = heating value of fuel M = number of heat level required pi = pressure of steam supplied for ith heat demand p n = nth cut-point which defines the feasible regions for

Q = cumulative heat demand Ri = ratio of recoverable heat to power for ith heat demand Rj = ratio of recoverable heat to power for jth steam demand ti = temperature of steam supplied for ith heat demand W = power demand xi = fuel quantity that may be consumed by furnace for ith

= fuel quantity that may be consumed for cogeneration

levels in a total heat demand Q

boiler

solutions

heat demand

of rth heat demand and power

xj+m = fuel quantity that may be consumed for cogeneration

~ 3 M + 1 = fuel quantity that may be consumed for power gen-

~ i + 3 ~ + 1 = electric power that may be consumed by electric

q b = boiler efficiency qc = overall thermal efficiency of condensing turbine q ~ i = efficiency of electric heater for ith heat demand qFi = furnace efficiency for ith heat demand TM = mechanical efficiency of steam turbine qTi = overall thermal efficiency of power generation using

turbine with exhaustive steam for heat demand qTj = overall thermal efficiency of power generation using

steam turbine with exhaustive steam for steam demand

Literature Cited Nishida. N.; Stephanopouias, G.; Westerberg A. AIChEJ. 1980, 27(3), 321. Nishio, M.; Itoh, J.; Shiroko K.; Umeda T. Ind. Eng. Cbem. Process Des.

Dev. W80, 19, 306. Nishio, M.; Shiroko K.; Umeda T. Ind. Eng. Chem. Process D e s . Dev. 1982,

21. 640. Nishio, M.; Shiroko K.; Umeda T. "Optimizatlon Problem of Chemical Pro-

cessing Systems", The 2nd Mathematical Programming Symposium, Kyo- to, Japan, Oct 1981.

Nishio, M.; Koshijima I.; Shiroko K.; Umeda T. Ind. Eng. Cbem. Process Des. Dev. 1984, 23, 450.

Umeda. T.; Itoh J.; Shiroko K. Chem. Eng. Progr. 1978, 74(7), 70. Umeda, T.; Shiroko K. In "Energy Utilization Engineering", (in Japanese) Ohm

Press: Tokyo, 1980; Chapter 5. Umeda, T.; tiarada T.; Shiroko K. Comput. Cbem. Eng. 1981, 3(1), 273.

of jth steam demand and power

eration by condensing turbine

heater for ith heat demand

Receiued for review September 17, 1982 Accepted January 23, 1984

Work was presented at the AIChE 91th National Meeting, Feb 1982.

Mild Cracking of High-Molecular-Weight Hydrocarbons

Byouk Blouri,' Fadl Hamdan, and Danklle Herault

Laboratolre de G n i e et Informetlque Chlmlques, Ecole Centrale des Arts et Manufactures, 92290 Chatenay-Makrlabry, France

Controlled cracktng in the liquid phase of nhexadecane, G m e t h y l e i n e , I-phenyfdodecane, and C2,-C2, paraffins was studied in a stainless steel microreactor between 350 and 440 O C for residence times varying from 0.5 to 4 h at nitrogen or hydrogen pressures of 20 bar. Cracking occurred according to a molecular mechanism, but its kinetic data such as the order of reaction and the activation energy were similar to those of a radical type cracking. The rate of formation of cracked gases was extremely small and the experimental and simulated compositions of the cracked liquids, based on a molecular type scission, agreed very well. This type of cracking is very interesting for visbreaking of heavy oils.

Introduction Industrial thermal cracking and, more specially, steam

cracking of hydrocarbons for the preparation of gaseous and liquids olefins occurs between 600 and 850 "C, ac- cording to a radical mechanism as proposed by Rice (1931), and defined more accurately by Kossiakoff and Rice (1943).

For the radical type cracking of light hydrocarbons, it has been shown for n-nonane (Kunzru et al., 19721, and for n-octane, isooctane, and cyclohexane (Illes et al., 1973) that the distribution of the cracked products could be predicted within 20%, according to Rice and Kossiakoff's theory. However, for steam cracking of high-molecular- weight hydrocarbons, such as n-tetracosane, 6-methyl- eicosane, and 1-phenyldodecane, Blouri et al. (1981)

showed that the difference between experimental and simulated results was much bigger.

At mild temperatures, 350-550 "C, in particular in the liquid phase, hydrocarbons can be cracked according to a molecular mechanism. For cracking in the gaseous phase of a-olefins into propene and another molecule of a-olefin, Miller (1963) proposed a mechanism of scission of the molecular type involving a six-membered ring. Mild cracking of 1- and 2-tetradecene in the gaseous phase oc- curs simultaneously according to both molecular and radical mechanisms (Giraud-Horvilleur and Blouri, 1977).

The present paper reports the experimental results of a kinetic and chemical study of thermal cracking occurring in the liquid phase between 350 and 440 "C for three high-molecular-weight hydrocarbons (with more than 16

0 1984 American Chemical Society

Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 1, 1985 31

T . 6

Figure 1. Experimental device.

carbon atoms): n-hexadecane, 6-methyleicosane, and 1- phenyldodecane. These results are compared with simu- lated molecular type cracking of these hydrocarbons.

Visbreaking of residues has been studied by several authors: Beuther et al. (1959), Rhoe and de Blingnicres (1979), Aiba et al. (1981), Akbar and Greelen, (1981), and Hus (1981). This latter work studied the mechanism of thermal visbreaking. All of these works concerned a mild cracking of heavy products carried out in the liquid phase at 400-550 "C in order to obtain lighter products of lower viscosity.

There is no fundamental study available on thermal cracking of higher-molecular-weight hydrocarbons occur- ring under pressure in the liquid phase. Studies on the mild cracking of a-olefins by Miller (1963), and the cracking of long-chain molecules by Giraud-Horvilleur and Blouri (1977) were carried out in the gaseous state. Experimental Section

Apparatus. The first cracking experiments were car- ried out in a 125-cm3 autoclave which was rocked and heated with an electrical furnace. This apparatus could be heated to a maximum temperature of 400 "C at a maximum pressure of 400 bar. Thus we could define zones of reaction temperatures, residence times, and more spe- cially nitrogen or hydrogen pressure threshold in order to maintain the product in the liquid phase.

The first drawback of this autoclave was the tempera- ture limitation to 400 "C. The second drawback was the high thermal inertia at the beginning of heating, and above all, the difficulty to obtain a rapid cooling at the end of the reaction. To cope with these difficulties, a stainless steel microreactor was constructed in the laboratory, as shown in Figure 1.

Reactor (1) was a tube of stainless steel 18/8,40/44 mm in diameter and 120 mm in length with a central sheath (2) consisting of a tube 26/30 mm in diameter. Two sheaths of chromel-alumel thermocouples (3) and (4) al- lowed the measurement of reactor temperature. The re- actor was heated by an electric resistor (not represented) a t its lower part (5) consisting in a steel block. Heating was regulated by an "on-off" system operated by the thermocouple (3). The product to crack was introduced into the tube (6) and the cracked products were evacuated through the Nupro valve (7). The carrier gas, either hy- drogen or nitrogen, was introduced through the Nupro valve (8) and divided at its outlet by the sintered plate (9). (a) In case of nitrogen, this carrier gas allowed us to ensure a constant pressure in the microreactor; this condition was absolutely necessary to maintain the hydrocarbon treated in the liquid state and to blend the product in the reactor. (b) In case of hydrogen, it could act both as carrier and reactant. The reactor was connected through the Nupro

valve (10) to a condenser (11). This condenser consisted in a tube of 120 mm in length and 26/30 mm in diameter. The condenser was plugged into a Dewar flask filled with ice (not represented). The light products could be evac- uated through the Nupro valve (12). The pressure of the whole system was indicated by the manometer (13). The flow rate of the carrier gas was regulated by the Nupro valve (14) and was read on the rotameter (15) after ex- pansion at a pressure close to atmospheric. During the experiment, nitrogen or hydrogen was introduced contin- uously at a flow rate of 1.5 L/h.

Analysis. The gases were analyzed by two gas chro- matographs, under operating conditions as previously described by Blouri et al. (1981).

The liquids were analyzed in a chromatograph (GIR- DEL), fitted with a flame ionization detector and a glass capillary column 25 m in length and 0.30 mm in diameter, whose stationary phase was SE 30. The operating con- ditions were: nitrogen flow, 3 cm3/min; injector temper- ature, 350 "C; detector temperature, 350 "C; column tem- perature, programmed from 50 to 230 "C at a rate of 3 "C/min.

Branched paraffins and monoolefins were eluted in the chromatograms between the normal paraffin and the next higher a-olefin. The chromatographs were connected to an integrator (DELSI) which gave the mass percentage of cracked products. The molar percentage reported in the tables was deduced from the mass percentage.

Treated Hydrocarbons. The 6-methyleicosane was prepared in the laboratory by allowing 2-heptanone to react with a magnesium derivative of l-bromo-n-tetrade- cane under the operating conditions described by Blouri et al. (1981). The n-hexadecane, the mixture of n-paraffins with 21 to 27 atoms and the 1-phenyldodecane with 99% purity were commercial products. The hydrocarbon purity was controlled by gas chromatography. Results

reaction rate (first order) is (1) Kinetic Analysis. For a reaction: A - vC the

- KCA (1) dCA dt

r = - - -

CAO CA

In - = Kt (3)

(5) In - = Kt

with a = fractional hydrocarbon conversion rate, t = residence time, and K = rate constant. For a first-order reaction, the graph of the natural logarithm of 1/(1- a) vs. residence time is linear. The intercept is the origin and slope of the rate constant K.

(a) Order of the Reaction and Rate Constant K . Figures 2, 3, 4, and 5 show the variation of the natural logarithm of 1/(1 - a) vs. the residence time for a small conversion. The graphs are linear through the origin. Reactions order in controlled cracking of hydrocarbons is therefore very close or equal to one. The slope at the origin of these straight lines is the rate constant K. The obtained

1 1-CX

32 Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 1, 1985

2.1

4 0 c -

y--+--- ' 3 : 2

Figure 3. Plots of In (1/(1 - a)] vs. residence time.

Figure 2. Plots of In [ l / ( l - a)] vs. residence time.

Table I rate constants, k . s-l

temp, 6-methyl- 1-phenyl- paraffins "C n-hexadecane eicosane dodecane C21-C27

350 - - 3.80 X lo4 - - 6.74 X lo4 - 360 -

380 - - 2.02 x 10-5 -

400 4.06 X lo4 9.19 X lo4 5.56 X -

420 1.51 X 3.11 X - 2.58 x 10-5 440 4.79 x 10-5 - - -

Table I1 E,. kcal A

n- hexadecane 59.2 3.09 x 1013 6-methyleicosane 56.5 2.49 x 1013 1-phenyldodecane 44.4 1.57 X 10'O

values are listed in Table I. (b) Activation Energy.

function From the graphs of the

In (Ki) = f(104/T) (6)

(function obtained according to Arrhenius's law), the values of the global activation energies (E,) and the preexponential factors (A) were deduced, as shown in Table 11. Except for the values reported by Blouri et al. (1981) for the activation energies of some high-molecu- lar-weight hydrocarbons (56.3 kcal for 6-methyleicosane, 45 kcal for 1-phenyldodecane), no other kinetic data are found in the literature.

(2) Composition of the Cracked Products. (a) n - Hexadecane. Table I11 gives the operating conditions and the molar distribution of the crakced products, in moles per 100 mol of cracked n-hexadecane obtained, at nitrogen or hydrogen pressures of 20 bar, for a residence time of

Figure 4. Plots of In [ l / ( l - CY)] vs. residence time.

4 h, the amount of n-hexadecane being maintained at 40 g for all experiments.

The cracked products consisted of two complete series of normal paraffins and a-olefins. The normal paraffins were present in very similar molar quantities. It is also

Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 1, 1985 33

ethylene and another radical CH3(CH2)5CH2CH2 - CH2=CH2 +CH3(CH2)3CH2CH2

CH3(CH3)3CH2CH2 - CH2=CH2 +CH&H2CH2CH2

CH3CH2CH2CH2 - CH2=CH2 + CH3CH2

The ethyl radical reacts with another n-hexadecane mol- ecule CH3(CH2)&H&H&H2CH2(CH2),CH, + CH3CH2 -

CHSCH3 + CH,(CH2),CH2CHZCHCH2(CH2)5CH, The secondary n-hexadecyl radical gives by 8-scission an a-olefin and another radical

CH3(CH2)&H&H2-CHCH2(CHZ)5-CH3 - CH2=CHCH2(CH2)&H3 + CH3(CH2)&H2

And the primary formed radical continues the 8-scission CH3(CH2),CH2 - CH2=CH2 + CH3(CH2)3CH2

(3) termination of the chain reaction by action of two free radicals

2CH3CH2 - CH3CH2CH2CH3

2CH3 - CH3CH3

In radical type cracking of paraffins, few liquids (chiefly a-olefins) and many gases (chiefly ethylene and propylene) are obtained.

In a molecular type cracking, mainly, liquid mixtures of a-olefins and paraffins, in a molar equivalent quantity, are obtained according to the following scheme

Figure 5. Plots of In [ l / ( l - a)] vs. residence time.

Table 111. Molar Product Distribution in Mild Thermal Cracking of n -Hexadwane (mo1/100 mol Cracked)

400, N2; 400; HZ; 440; N2; 440; HZ; 5.978a 5.600 39.42 39.05 2.404 0.962 0.955 0.956 0.393 0.562 0.391 0.564 0.393 0.563 0.392 0.564 0.392 0.563 0.391 0.565 0.392 0.152 t -

2.395 0.961 0.955 0.958 0.226 0.727 0.227 0.725 0.225 0.729 0.226 0.727 0.227 0.728 0.224 0.726 0.228 0.198 t t

16.58 6.566 6.579 6.576 1.495 5.085 1.491 5.087 1.494 5.084 1.497 5.089 1.489 5.081 1.492 5.091 1.502 1.068 0.498 -

16.57 6.558 6.569 6.574 0.892 5.673 0.888 5.680 0.885 5.668 0.886 5.689 0.879 5.645 0.875 5.650 0.877 1.282 0.169 0.334

The column headings indicate temperature, 'C, carrier gas, and conversion weight, % , respectively.

similar for the a-olefins, except for n-tetradecane and 1-pentadecene (resulting from the breaking of a primary bond and for n-pentadecane (resulting from the hydro- genation of 1-pentadecene).

Molecular Mechanism of Cracking and Compari- son of the Results of Simulated and Experimental Cracking. For a radical type cracking of a paraffin, e.g., n-hexadecane, according to Rice and Kossiakoff's theory (1943), and studies by Blouri et al. (198l), the thermal cracking decomposition is based on the following three reaction steps: (1) initiation by a C-C bond split CHB(CH2) bCH&H&H&H2( CH2) 6CH3 -+

2CH3(CH2)&H&H2

(2) propagation by 8-scission of the primary radicals giving

In the simulated calculation of cracking, dissociation en- ergies DHo of different bond types have been used: pri- mary C-C bonds (CH21CH3) 84 f 2 kcal, secondary C-C bonds (-CH21CH2-) 82 f 2 kcal, and tertiary C-C bonds (-CH21CH-) 80 f 2 kcal. The dissociation energy of C-H bonds (close to 100 kcal) depends on the carbon nature: 2 kcal between primary C-H and secondary C-H bonds, and 4 kcal between secondary C-H and tertiary C-H bonds.

We have taken by definition: (1) primary bond: split of a primary C-C bond and transfer of hydrogen from a secondary site to a primary site; split of a secondary C-C bond and transfer of hydrogen from a primary site to a secondary site; (2) secondary bond split of a secondary C-C bond and transfer of hydrogen from a secondary site to a secondary site; (3) tertiary bond: split of a tertiary C-C bond and transfer of hydrogen from a secondary site to a secondary site; split of a secondary C-C bond and transfer of hydrogen from a tertiary site to a secondary site. Moreover, in the ratio exp(AE/RT), AE is the difference of the dissociation energy. The following mean values were taken: AI3 = 2 kcal between a primary and a secofidary bond, and hE 4 kcal between a primary and a tertiary bond.

For predicted distribution of each compound issued from the split of a primary, secondary, or tertiary bond, an affected relative coefficient, calculated from the ratio exp(AE/RT) is given. For a product produced by breaking a: primary bond, AE = 0 cal; secondary bond, A E = 2000 cal; tertiary bond, A E = 4000 cal. E.g., for a hydrogen treated at 400 "C, this coefficient is: exp(O/RT) = 1;

34 Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 1, 1985

Table IV. Comparison of Predicted and Experimental Distribution of Cracking Products of n -Hexadecane (mo1/100 mol Cracked)

cracking products

c1-c4

1-C5H10 + C5H12 1-C6H12 + C6H14 1-C7H14 + C7H16 1-C8H16 + C8H18 l-CgH1, + C9Hzo ~-C,oHzo + C10H22 1-CllHzz + Cl1Hz4 1'C12H24 + C12H26

1-C13H26 + C13H28

1-C14H28 + C14H30 1-C15H30 + C15H32

exptl 41.46 16.59 16.47 16.49 16.46 16.46 16.49 16.48 16.47 16.48 9.30 t

simulated 43.75 16.06 16.06 16.06 16.06 16.06 16.06 16.06 16.06 16.06 9.83 1.80

exp(2000/1.987 X 673) = 4.46; exp(4000/1.987 X 673) = 19.91 for each molecular species which can be produced from the cracking of the given feed (respectively split of a primary bond, secondary bond, and tertiary bond).

According to the molecular mechanism, for 100 mol of cracked fresh feed and with a conversion rate, 2a moles are formed by thermal cracking. So, if x , y, and 2 are respectively the molecular species obtained by the cracking of primary bonds, secondary bonds, and tertiary bonds in the given feed, the number of moles produced for each molecular specie is for product coming from a primary bond split

2a x + 4 . 4 6 ~ + 19.912

product coming from a secondary bond split

4.46 X 2a x + 4 . 4 6 ~ + 19.912

product coming from a tertiary bond split 19.91 x 2a

x + 4 . 4 6 ~ + 19.912

In the cracking of n-hexadecane, 28 compounds from methane to 1-pentadecene are formed; 24 compounds are produced from the breaking of secondary bonds and 4 compounds from the breaking of primary bonds, according to the scheme in eq 11. The scission according to the

'ZH4 + Of 'ZH6 + C14H28

H k H H Y H H

(11) I I I I I I I I I I

I I I I I l l l l l H-CfC+C-(CH.&-Cf C t C-H

H H/ H H \H H

CH, t l-C15H3G

scheme 1 or 4 will lead to the breaking of a primary C-C bond and the breaking and transfer of hydrogen from a secondary carbon to a primary carbon. In the scission 2 or 3, there are two possibilities: (1) breaking of a secondary C-C bond, breaking and transfer of hydrogen from a primary carbon to a secondary carbon, giving ethylene and n-tetradecane; (2) breaking of secondary C-C bonds and secondary C-H bonds, giving ethane and 1-tetradecene. According to the rates of formation listed above, the sim- ulated cracking of n-hexadecane, exclusively molecular type, was calculated for 400 "C, and the results are listed, with the experimental results of its cracking, a t 400 "C, with nitrogen, for a residence time of 4 h, and a conversion rate of 5.6%) in Table IV.

Table V. Molar Product Distribution in Mild Thermal Cracking of 6-Methvleicosane (mo1/100 mol Cracked)

400; N2; 400; H2; 420; N2; 420; HZ; 11.86' 11.89 32.01 32.09

C,-Ca 3.076 3.085 6.462 6.462

0.330 1.482 0.306 0.154 0.309 0.162 0.305 0.157 0.316 0.153 0.308 0.164 0.307 0.152 0.319 0.161 0.311 0.155 0.314 0.158 0.307 0.156 1.165 0.158 0.306 1.070 0.925 0.160 0.313 0.162 0.307 2.012 0.328 0.607 0.321 0.612 0.311 0.125 0.081

0.051 -

-

2.397 0.488 2.331 0.341 1.371 0.390 0.088 0.393 0.089 0.397 0.090 0.402 0.094 0.393 0.093 0.389 0.088 0.396 0.091 0.396 0.092 0.392 0.090 0.393 0.091 1.237 0.089 0.388 0.723 1.196 0.092 0.401 0.094 0.394 2.009 0.197 0.773 0.183 0.776 0.225 0.202 0.055 0.028 0.033 t

7.004 1.306 6.235 0.839 3.943 0.935 0.377 0.928 0.375 0.942 0.382 0.924 0.369 0.930 0.365 0.932 0.378 0.928 0.373 0.926 0.374 0.931 0.368 0.936 0.365 3.257 0.362 0.939 2.841 2.879 0.371 0.921 0.376 0.917 5.735 0.704 1.839 0.727 1.815 0.732 0.446 0.178

0.146 -

-

7.008 1.310 6.241 0.857 3.751 1.098 0.207 1.134 0.212 1.115 0.209 1.090 0.202 1.114 0.200 1.101 0.206 1.096 0.204 1.092 0.197 1.108 0.201 1.104 0.195 3.451 0.198 1.108 2.106 3.607 0.203 1.087 0.206 1.082 5.711 0.383 2.157 0.398 2.141 0.406 0.795 0.122 0.069 0.101 0.057

' The column headings indicate temperature, "C, carrier gas, and

(b) 6-Methyleicosane. The operating conditions and molar distributions of the cracked products of 6-methyl- eicosane are listed in Table V. Cracking occurred at a pressure of 20 bar of nitrogen or hydrogen, for a residence time of 4 h. The amount of 6-methyleicosane used for all experiments was 40 g. The cracked products are formed of (a) n-paraffins from methane to hexadecane, except pentadecane. Tridecane, tetradecane, and hexadecane are in important quantity, while the molar quantity of another paraffins are similar; (b) a-olefins from ethylene to tet- radecene, but also eicosene; 1-tetradecene appears bigger in molar quantity than the others, while eicosene is very slight; (c) branched paraffins from methylheptane to me- thyloctadecene; the rate of methylhexadecene or me- thylheptadecane is twice the rate of other branched pa- raffins; (d) branched olefins from methylheptene to me- thylnonadecene. The rate of methylhexadecene, me- thylheptadecene, or methyloctadecene is twice the rate of other branched olefins.

Comparison of the Results of Simulated and Ex- perimental Cracking. As for n-hexadecane, the pre- dicted composition of cracked products of 6-methyl-

conversion weight, % , respectively.

Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 1, 1985 35

Table VI. Comparison of Predicted and Experimental Distribution of Cracking Products of 6-Methyleicosane (mo1/100 mol Cracked)

cracking products exptl simulated 25.94 27.07 20.19 20.29 04.09 03.71 21.90 20.29 15.08 13.85 07.84 07.42

07.88 07.42

07.94 07.42

07.93 07.42

15.06 13.85

20.73 20.28

03.99 03.71 20.92 20.28

07.88 13.85 07.87 07.42 03.68 04.54 01.11 01.25

eicosane has been based on the number of compounds formed from every breaking type, and on their bond en- ergy.

6-Methyleicosane has 3 primary C-C bonds, 15 sec- c H3

1 2 3 4 5 I b T 8 9 IS 20 C H ~ - C H ~ - C H ~ - C H ~ - C H ~ - C H - C H ~ ~ H ~ - ( C H ~ ) K ) - C H ~ - C H ~ (12)

ondary C-C bonds, 2 tertiary C-C bonds, 9 primary C-H bonds, 34 secondary C-H bonds and 1 tertiary C-H bond. The bonds 1,6, and 20 are considered primary C-C bonds, the bonds 5 and 7 are considered tertiary C-C bonds, and the other bonds are secondary C-C bonds. The C-H bonds of three methyl groups are primary, the C-H bond of tertiary carbon is tertiary, and the other C-H bonds are secondary.

The simulated cracking of 6-methyleicosane was based on the formation of 10 products issued from the breaking of a primary bond, 52 products issued from the breaking of a secondary bond, and 12 products issued from the breaking of a tertiary bond. Based on these data, the simulated cracking of 6-methyleicosane was calculated according to an exclusively "molecular type" mechansim, and the simulated and experimental results of cracking, with nitrogen, a t 400 "C, for a residence time of 4 h and a conversion rate of 11.86%, are listed in Table VI.

(c) l-Phenyldodecane. Operating conditions and molar distribution of the cracked products of l-phenyl- dodecane are listed in Table VII. Cracking occurred at a nitrogen or hydrogen pressure of 20 bar, for a tempera- ture of 400 "C. The amount of used l-phenyldodecane was maintained at 40 g for all experiments.

At temperatures under 400 "C, the cracked products formed were exclusively toluene, styrene, n-decane, and l-hendecene. Toluene and l-hendecene resulted from the molecular breaking of the C-C bond near the benzene ring, of bonding energy very low when compared to the others (67 kcal). The mechanism involves a four-membered ring similar to the six-membered one in Miller's hypothesis. At 400 "C, a scission of the lateral chain also occurred with formation of paraffins, a-olefins up to Clot alkylbenzenes, and alkenylbenzenes.

Table VII. Molar Product Distribution in Mild Thermal Cracking of l-Phenyldodecane (mo1/100 mol Cracked)

2; Nz; 2; Hz; 4; N2; 4; HZ; 29.54' 29.65 44.34 44.54 1.267 0.476 0.473 0.474

0.324 0.152 8.166 0.325 0.325 0.151 0.151 0.325 0.324 8.166 0.152 0.324

17.48

17.50 - 0.151 0.325 0.152 0.324 0.151 0.324 0.151 0.325 0.152 0.324 0.151 0.065 t -

1.269 0.475 0.473 0.476

0.251 0.226 3.493 4.995 0.254 0.225 0.116 0.360 0.220 8.272 0.118 0.356 8.144 9.348 0.117 0.358 0.115 0.362 0.116 0.360 0.115 0.360 0.113 0.362 0.123 0.095 t t

17.51

2.105 0.782 0.782 0.784

0.537 0.251

0.536 0.537 0.250 0.250 0.537 0.536

0.251 0.537

25.85

12.08

12.08

25.85 - 0.251 0.536 0.251 0.536 0.251 0.536 0.250 0.536 0.250 0.536 0.251 0.112 t -

2.111 0.779 0.780 0.783

0.361 0.426 4.506 8.111 0.358 0.430 0.172 0.613 0.263

0.171 0.616

25.85

12.35

11.06 14.78 0.175 0.612 0.173 0.614 0.170 0.616 0.172 0.615 0.174 0.614 0.124 0.252 t t

The column headings indicate residence time, h, carrier gas, and conversion weight, respectively.

(d) C 2 1 4 2 7 Paraffins. Table VI11 shows the molar distributions of the cracked products, expressed in moles per 100 moles of paraffins cracked, for a temperature of 420 O C , a nitrogen or hydrogen pressure of 20 bar, and a residence time of 2 and 4 h. The amount of used paraffins was maintained at 40 g for all experiments.

The cracked products of the paraffins consisted exclu- sively of the complete series of n-paraffins and a-olefins. Except for the initial paraffins, the n-paraffins were present in very similar molar amounts. The same holded for the a-olefins. A comparison of the ctacked products of n-hexadecane with those of C21-C27 paraffins permitted us to extend the results of the cracking of these long chain hydrocarbons to all heavy paraffins (more than 27 carbon atoms).

Influence of Hydrogen. It clearly appeared from the compositions of the cracked products of n-hexadecane, 6-methyleicosane, l-phenyldodecane, and C21-C27 paraf- fins, reported in Tables 111, V, VII, and VIII, that hydrogen presence increased the paraffinsla-olefins ratio and the alkylbenzenes/alkenylbenzenes ratio (Figures 6 and 7).

To determine more accurately the effect of the carrier gas pressure, three cracking experiments were carried out with 6-methyleicosane at 400 "C, with a residence time of 4 h in the presence of nitrogen or hydrogen, at a pressure varying between 20 and 68 bar. The results are listed in Table IX. The ratios of the n-paraffinsla-olefins or iso- paraffins/isoolefins remained constant when the carrier gas was nitrogen but increased as a function of the hy- drogen pressure. Conclusions

The mild cracking in the liquid phase of n-hexadecane, &methyleicosane, l-phenyldodecane, and C21-Cn paraffins

36 Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 1, 1985

te t A A

AI 1-C5H,, + C16H3, or C,H12 + 1-CI6H,,

Table VIII. Molar Product Distribution in Mild Thermal Cracking of Ca-Cn Paraffins (mo1/100 mol Cracked)

3.65 1.89 1.84 0.55 1.45 0.56 1.37 0.54 1.43 0.51 1.40 0.53 1.34 0.57 1.46 0.55 1.45 0.56 1.46 0.53 1.32 0.55 1.40 0.58 1.43 0.56 1.44 0.53 1.41 4.76

12.02 19.56 21.98 17.76 12.64 7.01

4.02 1.88 1.92 0.44 1.47 0.42 1.45 0.43 1.50 0.44 1.49 0.45 1.52 0.44 1.55 0.45 1.55 0.43 1.60 0.46 1.56 0.45 1.58 0.45 1.56 0.44 1.56 0.42 1.58 4.83

11.85 19.14 22.34 17.60 12.71 6.95

6.78 3.40 3.53 1.02 2.60 0.97 2.49 0.98 2.60 1.01 2.48 1.00 2.62 0.96 2.58 0.97 2.42 1.03 2.56 0.97 2.51 0.96 2.60 0.98 2.63 0.99 2.52 1.02 2.55 5.29

10.74 17.38 18.57 14.78 10.17 5.80

7.23 3.40 3.45 0.54 3.03 0.52 3.03 0.52 3.00 0.54 3.02 0.54 3.00 0.54 2.97 0.52 2.99 0.56 3.01 0.54 2.99 0.53 2.95 0.54 2.96 0.53 2.94 0.54 2.95 5.37

10.95 17.01 18.88 14.65 10.24 5.75

The column headings indicate residence time, h, carrier gas, and conversion weight, %, respectively.

Table IX. Effect of Pressure on the Ratios pressure, bar 20 20 45 45 68 68

conversion weight, 11.86 11.89 11.93 11.91 11.85 11.99

n-paraffins/ 2 4.46 2 5.30 2 6.26

isoparaffins/ 2.01 4.47 2.02 5.29 1.99 6.23

carrier gas Nz Hz Nz Hz N2 H2

%

a-olefins

isoolefins

(representing three families of isoparaffinic, aromatic, and paraffinic hydrocarbons of petroleum oils, occurred ac- cording to a molecular type mechanism. This is perfectly clear, because according to the Rice and Kossiakoff's radical mechanism, we should obtain gaseous mixtures with chiefly ethylene and propylene, and liquids mixtures with exclusively a-olefins.

In the cracking of n-hexadecane and C21-C2, paraffins, gases are in small quantities, and liquids are formed by a mixture of paraffins and olefins, according to the mo-

- Ratio

r-Hexadecane

b w i t h N 2

0 w i t h H 2 6.

5 1 I

I

4 G O ' C 7

Residence tire. h

Figure 6. Ratio of n-paraffinsla-olefins and isoparaffins/isoolefins vs. residence time.

lecular mechanism scheme (7). According to this mecha- nism, for a normal long-chain paraffin such as n-hexade- cane, the results of simulated and experimental cracking agree very well, except for the C1-CI paraffins. For this product we obtained an experimental value of 41.46 mo1/100 cracked mol, but a simulated one of 43.75 mol. The difference is very small and is due to experimental difficulties in trapping these light paraffins.

For radical type cracking of n-tetracosane, Blouri et al. (1981) have shown that the difference between simulated and experimental results are very important. Cracking of 6-methyleicosane is also a molecular type mechanism; the presence of a methyl branched group enhances the for- mation of some hydrocarbons, schematized in eq 13. For every breaking of C-C bonds of 6-methyleicosane, there occurs formation of 4 products, except for primary C-C bonds, where we have formation of 2 products. Thus the breaking of CTC and CTC bonds, according to the scheme B, will lead to the split of a secondary C-C bond and a transfer of hydrogen from a secondary site to a secondary site, when the scheme A will lead to the split of a secondary C-C bond and transfer of hydrogen from a tertiary site to

Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 1, 1985 37

molecular type, but due to the influence of the benzene ring, a t temperatures in the range of 350-380 “C, toluene and styrene are obtained selectively together with n-decane and 1-hendecene. At 400 “C the complete series of scission products of the lateral chain of 1-phenyldodecane is ob- tained besides these hydrocarbons.

Cracking in the liquid phase under hydrogen pressure gives similar results, but part of the formed olefins are hydrogenated into the corresponding paraffins. An in- crease in the hydrogen pressure enhances formation of saturated hydrocarbons.

Practically, visbreaking of paraffins, isoparaffins, and aromatic hydrocarbons with long lateral chains, occurring according to a “molecular type” mechanism, can lead to light hydrocarbons. The presence of hydrogen (hydro- visbreaking) increases the paraffins/olefins ratio and sta- bility of cracked liquids. Our research work has been pursued by the study of visbreaking and hydrovisbreaking of “saturated” and “aromatic” fractions in heavy petroleum oils. Acknowledgment

This study was carried out within the action and fi- nancial assistance of General Delegation for Scientific and Technical Research of the French Ministry for Research and Industry on “Assisted Recovery of Petroleum”.

Registry No. n-Hexadecane, 544-76-3; 6-methyleicosane, 65848-36-4; 1-phenyldodecane, 123-01-3; 2-heptanone, 110-43-0; 1-bromo-n-tetradecane, 112-71-0.

Literature Cited Aiba, T.; Kaji, H.; Suzuki, T.; Wakamatsu, T. Chem. Eng. Prog. 1981, 3 7 , 2 . Akbar, M.; Greelen, H. Hydrocarbon Process. 1981, 5 , 81. Beuther, H.; Goldhwait, R. G.; Offutt, W. C. Oil Gas J . 1959, 57(46), 35. Blouri, B.; Giraud, J.; Nouri, S.; HBrauk, D. Ind. Eng. Chem. Process Des.

Glraud-Horvllleur, F.; Blourl. B. Inf . Chim. 1977, 764, 113. Hus, M. 011 Gas J . 1981, 73(4), 109. Illes, V.; Welther, K.; Pleskats, I. Ac. Chim. (Budapest) 1973, 76(4), 357. Kossiakoff, A.; Rice, F. 0. J. Am. Chem. Soc. 1943, 65, 520. Kunzru, D.; Shah, Y. T.; Stuart, E. B. Ind. Eng. Chem. Process Des. Dev.

Miller, D. B. Ind. Eng. Chem. Process Des. Dev. 1963, 2, 220. Rhoe, A,; de Bligni&es, C. Hydrocarbon Process. 1979, 7 , 13 1. Rice, F. 0. J . Am. Chem. SOC. 1931, 53, 1959.

Dev. 1981, 20, 307.

1972, I f , 605.

Received for review December 21, 1982 Revised manuscript received June 3, 1983

Accepted December 6, 1983

6-Methyleicosane I /

4 2 0 ’ C i

400‘C t

~ e s l d e n c e time, h

0.5 1 2 4

Figure 7. Ratio of n-paraffiislci-olefins and isoparaffins/isoolefns vs. residence time.

a secondary site. The breaking of CTC and CTC bonds, according to the two schemes A and B will lead to the split of a tertiary C-C bond and transfer of hydrogen from a secondary site to a secondary site. The scission products of other bonds in 6-methyleicosane are similar to the ones from n-paraffins.

The results of simulated and experimental molecular cracking of 6-methyleicosane nearly agree except for CI7 hydrocarbons, but if we consider the breaking 4 (scheme A) of 6-methyleicosane as secondary (there is probably transposition of branched methyl group instead of breaking of the tertiary C-H bond), the simulation of CI7 hydro- carbons will agree with the experiment.

For an aromatic hydrocarbon with long lateral chain (1-phenyldodecane), the scission mechanism is also of the