CC BY
1УДК 62 BabayevR.K., TamiliN.T.
Babayev R.K.
Ph.D., Associate Professor Azerbaijan State University of Petroleum and Industry (Baku, Azerbaijan)
Tamili N.T.
Master's degrstudent Azerbaijan State University of Petroleum and Industry (Baku, Azerbaijan)
STUDY OF THE DEHYDROGENATION PROCESS OF HIGHER NORMAL PARAFFINIC HYDROCARBONS FOR COMPUTER MODELING
Аннотация: the oxidative dehydrogenation of n-paraffin hydrocarbons represents a promising approach for producing linear olefins. Conducting the dehydrogenation of higher n-paraffins in the presence of oxygen helps eliminate the nonstationary and cyclic nature of traditional processes. The current focus on this method stems from the necessity to develop higher olefin production utilizing readily available raw materials. This article examines the kinetic patterns of the oxidative dehydrogenation process for higher n-paraffinic hydrocarbons. The resulting kinetic model was analyzed computationally across a broad range of parameters, providing insights into how the primary parameters (Ci) and process characteristics evolve with contact time (t).
Ключевые слова: dehydrogenation, paraffins, catalyst, olefins, kinetics, n-decane, non-stationarity, atmospheric oxygen, water vapor, model.
The dehydrogenation of higher normal paraffins in the presence of oxygen represents a promising approach for producing higher olefinic hydrocarbons. These hydrocarbons find significant application in the production of surfactants used in anionic synthetic detergents (alkylbenzene sulfonates) with biodegradability
exceeding 90%. Due to their high reactivity, higher olefinic hydrocarbons are extensively utilized in various sectors of the economy, including as active components in synthetic detergents, oil additives, and corrosion inhibitors [1].
Dehydrogenation in the presence of oxygen eliminates the non-stationary and cyclic nature of conventional processes. Current research focuses on this method to establish the production of higher olefins based on readily available raw materials. Presently, surfactant production primarily relies on alkylbenzenes synthesized by alkylating benzene with chlorinated paraffins or kerosene. This process releases hydrogen chloride and chlorine into the environment, causing ecological harm. Utilizing olefinic hydrocarbons derived from paraffin dehydrogenation in surfactant production enables the creation of an environmentally sustainable process.
Kinetic studies of the reaction were conducted in a laboratory-scale reactor under gradient-frconditions at temperatures ranging from 813-853 K. The initial molar concentrations of n-decane and oxygen were varied within the ranges of (2.537-5.075) x 10 4 mol/L and (0.374-1.869) x 10 4 mol/L, respectively, with contact times not exceeding 0.12 s. The catalyst employed was a nickel-antimony-vanadium oxide system modified with lithium oxide, supported on AI2O3. The specific surface area of the catalyst, determined via low-temperature nitrogen adsorption and calculated using the BET method, was 80-100 m2/g with a bulk density of 0.873 g/cm3 and tablet sizes of 2-3 mm.
Gas analysis for CO2 and low-molecular-weight hydrocarbons (Ci-Ce) was performed using an LHM-80M chromatograph equipped with a 6 m column. The same instrument, using a 3 m column filled with NaX zeolite, was used to determine H2, CO, O2, and CH4. Paraffinic and olefinic hydrocarbons were analyzed with a Tsvet-100 chromatograph, and aromatic hydrocarbons from the dehydrogenation process were characterized using a chromatograph-mass spectrometer.
The study investigated the effects of varying concentrations of reactants, target products, and by-products on the reaction rates, paraffin hydrocarbon conversion, and oxygen consumption. It was found that increasing the paraffin
hydrocarbon concentration from 2.148 x 10~4 to 3.745 x 10_4 mol/L enhanced the formation rates of both target and by-products. Concurrently, the oxygen concentration decreased significantly, attributed to rapid consumption at the catalyst bed's inlet. The maximum oxygen consumption rate was observed at a paraffin concentration of 3.745 x 10 ^ mol/L, with nearly complete oxygen conversion within the catalyst volume.
The influence of the target product (decene) concentration on its formation rate was examined within a range of 0 to 0.7159 x 10 mol/L at a constant contact time of 0.04 s, maintaining fixed oxygen and paraffin concentrations. Increased olefin concentrations in the feedstock led to higher CO2 formation rates, indicating partial combustion of olefinic hydrocarbons in the oxygen flow.
Introducing CO2 into the reaction products reduced the hydrocarbon combustion rate within the catalyst, freeing oxygen for coke deposit removal from the catalyst surface. This redistribution increased the catalyst's active surface area. Changes in hydrogen concentration in the feedstock had minimal impact on paraffin hydrocarbon conversion rates or product formation. However, oxygen consumption during coke burning reduced the target product's formation rate. A decline in olefin hydrocarbon production corresponded to decreased aromatic hydrocarbon formation.
Raising oxygen concentration in the catalyst bed reduced coke deposition, enhancing hydrocarbon cleavage rates. The presence of paraffinic and olefinic hydrocarbons, CO2, H2, and oxygen in the reaction mixture did not inhibit the formation rates of olefinic hydrocarbons.
The experiments revealed the reaction's rate-limiting stage under the studied conditions. Tests varying catalyst grain diameters (1.5-3 mm) and mixture flow velocities (0.13-0.53 m/s) showed consistent results under identical conditions, confirming negligible influence of external and internal diffusion on the reaction rate.
Based on the obtained kinetic data, a set of independent reaction pathways was identified for further optimization:
^n^2n+2 + °>502 ^ CnH2n + H2O CnH2n+2 + 0,5(1 + 3n)02 ^ nC02 + (n + 1)^0 CnH2n + 1,5n02 ^ nC02 + nH20
Cn^2n+2 ^ 5 C1- C5)
CnH2n+2 + 2nH20^nC02 +(3n + 1)H2
Accordingly, the reaction rates along all routes are determined by the following systems of differential equations:
dr
dCi _ (k ncainC + k • ca2n • C^ + к ca4,n + к • са5 и + к ca6n )
^ 2,n ^ 4,n 1 ^ Л5п u ^ лб,п 1 /
Г/О
2 _ к C a,n C n к C a3 n C в'П
j _/t1,nu 1 л3.п u2
dr
dC 3 r ra5tn
J 5,n 4
dr
dC4- _ 5к C a*.n
~ 4,n 4
dz
\l Ca2.n re2,n .J, na3, n nh, n , L ^26,n
Г2.Л 1 Ч + /C3,nU 2 U1 +/Сб,Л 1 /
dC 5
-_ n
dT
dC б _
_ 4к5,n 1 V" 1 VM^ 1
dT
dC
б-4к 5,n • c;5,n +(3n+1)к Mc;6,n
dT
_-0.5
k1n • Cain • C+(3n + 1)к2n - С" - C^ + 3nk3nC2a3,n • C^
dC_к • c;1," • cв +(n+1)к2.n • c^ • c!* + kjclxn • C^
dT
Where C1, C2 ,C7 - represent the concentrations of n-paraffin, olefin, and oxygen, respectively, and ki,n, ai,n ,P i,n correspond to the reaction rate constants and the reaction orders with respect to individual components (i=1-6; n=10-13).
It is assumed that the temperature dependence of the rate constants follows the Arrhenius law:
Ki = K0i-exp(-E/RT)
Where K0i are pre-exponential factors; Ei is the activation energy; R is the universal gas constant; T is the temperature.
The system was solved using a modified Runge-Kutta method [2]. The rate constants for individual reaction pathways were determined from the experimental curves.
Table 1 presents the experimental data showing the dependence of the concentrations of products formed during the dehydrogenation of n-decane on contact time at temperatures of 813 K, 833 K, and 853 K.
Table 1. The effect of contact time on the yield of ethylbenzene conversion
products (mol/l) on a modified oxide catalyst at various temperatures.
Compo nents Т=813 К Contact time. sec. Т=833 К Contact time. sec. Т=853 К Contact time. sec.
0,01 0,02 0,03 0,01 0,02 0,03 0,01 0,02 0,03
C10H22 3,721 3,539 3,439 3,578 3,489 3,369 3,546 3,433 3,295
C10H20 0,147 0,191 0,283 0,213 0,232 0,341 0,186 0,274 0,394
H2 0,035 0,097 0,144 0,112 0,129 0,192 0,143 0,185 0,258
O2 0.002 0,083 0,761 0,765 0,697 0,569 0,654 0,549 0,267
In the catalytic dehydrogenation of higher paraffins, steam is utilized to reduce the partial pressure of reaction components and serves as a heat carrier for the endothermic dehydrogenation reaction of n-decane. Additionally, steam facilitates the removal of coke deposits from the catalyst surface, helping to maintain catalyst activity.
The kinetic parameters were evaluated using a computer-based steepest descent method.
Table 2 provides the experimental values of the rate constants and the apparent activation energies for the process.
Table 2. Calculated values of kinetic parameters.
Rate constants, sec-1 Temperature. К Ei kcal/mol Koi , sec
813 833 853
Ki 0,0112 0,01511 0,02 20,075 0,2797663*104
K2 0,104 0,220 0,452 50,628 0,424524*1013
Кз 0,00058 0,00099 0,00165 36,199 0,3117592*107
K4 0,01479 0,02175 0,03141 25,943 0,1394821*106
K5 0,00726 0,0094 0,01341 21,142 0,3503986*104
Кб 0,00501 0,00724 0,01029 24,808 0,2339133*105
The obtained kinetic model of the n-decane dehydrogenation process in the presence of atmospheric oxygen was studied on a PC in a wide range of parameter variation and allowed us to obtain the dependence of the change in the main parameters (Ci) and process indicators on the contact time t.
The adequacy of the kinetic model was carried out by minimizing the difference in the squares of the deviation of the experimental and calculated ones presented in Table 2.
N m
F
=
j = 1 i=1
rexp _ ra
calc
Pexp Lij
^ min
Where C^. and Cff1^ represent the experimental and calculated
concentrations of the reactants, target products, and by-products, respectively, and a are the weighting coefficients.
The obtained kinetic parameters demonstrated good agreement between the calculated and experimental data, with a relative error of 5-1% for the target product and no more than 10% for the by-products.
Conclusions.
The kinetic laws of the process of oxidative dehydrogenation of higher n. paraffin hydrocarbons in a gradient-frreactor were studied. A scheme of the reaction mechanism was selected and the parameters of the kinetic model of the process were identified, adequately describing the experimental data. The obtained kinetic model of the process allows us to solve optimization problems in finding the optimal temperature regime of the reactor, to develop a dynamic model of the reactor, for the purpose of automatic control of the process
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