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CHEMICAL PROBLEMS 2022 no. 1 (20) ISSN 2221-8688
UDC 549.67: 544.47: 54.07:66.011
OPTIMAL DESIGN OF THE OXIDATIVE DEHYDROGENATION OF METHYLCYCLOHEXANE INTO METHYLCYCLOHEXADIENE ON A MODIFIED
ZEOLITE CATALYST
A.I. Karimov
Acad. M.F. Nagiyev Institute of Catalysis and Inorganic Chemistry,
National Academy of Sciences of Azerbaijan H. Javid ave., 113, Baku AZ1143, e-mail: kerimov. alibala@,mail. ru
Received 18.12.2021 Accepted 21.02.2022
Abstract: Selection and theoretical optimization of the reactor type was carried out on the basis of a kinetic model of the process of selective oxidative dehydrogenation of methylcyclohexane into methylcyclohexadiene on a modified active metal-zeolite catalyst. It was determined that it was more expedient to carry out the process in an ideal tubular (packed-bed) type reactor. As a result of theoretical optimization of the process, optimal technological regimes were determined and the optimal design dimensions of the reactor element for a given capacity calculated. A complete mathematical model of the process was developed with regard to the effect of heat and pressure drop.
Keywords: methylcyclohexane, methylcyclohexadiene, oxidative dehydrogenation, clinoptilolite, reactor choice, theoretical optimization, mathematical model. DOI: 10.32737/2221-8688-2022-1-48-58
Introduction
Methylcyclohexadiene is widely used in the production of synthetic jet fuels. At present, the process of dehydrogenation of methylcyclohexane has not been studied properly. The development of dehydrogenation catalysts with high stability, catalytic activity and product selectivity at low temperatures and high pressures is a key issue in the industrialization of the dehydrogenation process of methylcyclohexane [1] . In this respect, the synthesis of the valuable product -methylcyclohexadiene-1,3 by oxidative dehydrogenation of methylcyclohexane is of great theoretical and practical importance [2]. Given certain shortcomings of industrial methods for obtaining this compound, the laboratory of the ANAS Institute of Catalysis and Inorganic Chemistry developed an effective zeolite catalyst for the oxidative dehydrogenation of methylcyclohexane into methylcyclohexadiene. It established that
natural clinoptilolite containing 0.5% Co2+ and 0.25% Cr3+ exhibits relatively high activity in this reaction [3,4]. With the participation of this catalyst, the kinetic regularities of the process were studied and, on their basis, a theoretically substantiated kinetic model developed [4].
In the real devices, a chemical reaction is accompanied by physical processes and proceeds in the presence of hydrodynamics, heat and mass transfer. Thus, the chemical kinetics of the reaction does not fully reflect the real process. Therefore, macrokinetic factors must be taken into account by developing a mathematical model for the implementation of this process on an industrial scale.
The main purpose of this article is to address the issue of optimal design of the process under consideration. This, in turn, implies the selection of the reactor type based on the kinetic model, theoretical optimization, calculation of the optimal design dimensions of
CHEMICAL PROBLEMS 2022 no. 1 (20)
www.chemprob.org
the reactor element in accordance with the given mathematical model of the process. capacity and the creation of a complete
Choice of the optimal type of reactor
The choice of the optimal reactor type for the process under consideration was made by comparing the volumes required to obtain a given conversion rate in different reactors based on its kinetic model. There are two basic types of ideal reactors, stirred tanks and tubular or packed-bed reactors [5]. The process was
studied in these two types of reactors. The kinetic model of the oxidative dehydrogenation process of methylcyclohexane into methylcyclohexadiene on modified zeolites can be written as follows for an ideal tubular ( packed-bed) type reactor:
dA1
/ \ G
d
0
c7h
n, m
V C7H14 y
k11pc7h14
dA?
- №
( k3PC7Hi4
G^
V n C7 H14 y
k3PC7Hi4
kiPo2
+
k3PC7Hi4
k2PO
+ .
'2 7
k3PC7Hi4
kiPo2
+
k3PC7Hi4
k2PO
+ 4-
kP
3PC7Hi4
2 7
k
4
,k3PC7Hi4
dA
- - koPn
d
f — - k8P C7H14
Gk
0
CH,
Vn C7H14 )
k8PC7H14 k5Po9
k8PC7H14 k6PO9
k8PC7H14 k5Po,
k8PC7H14 k6PO9
k8PC7H14
k7P,
7PO
+ 4-
kP
'2 y
8 C7H14
tT
k8PC7H14
2
2
2
d
k
4
2
2
+
+
+
+
+
k
9
dA,
nCH
V C7H14 y
(1+K1P1
k13K1P1K6P2
(1 + K1P1 +JK
1+KP +,/K2P2 + K3P3 + K4P4 + K5P5 + K6P2
k14K3P3K6P2
1 + K,P + ,/K2P2 + K3P3 + K4P4 + K5P5 + K6P2
7 (1+K1P1 +VK
k15K4P4K6P2
[ + K,P + ,/K2P2 + K3P3 + K4P4 + K5P5 + K6P2
For an ideal mixing type reactor, the kinetic model of the process can be written as follows:
+
2
d
2
A
G,
- k11PC7Hj4
nC H
V C7H14 7
dA
d
G.
^ - k3PC7Hi4
nC H
V C7H14 7
'k3PC7H1^ |k3PC7H14
V. k1PO2
k2PO
'k3PC7H1^ |k3PC7H14
\2
2 7
^ k1PO2
k2PO
+ 4
27
k3PC7H!4 k4
2
k3PC7H!4 k4
dA3
- k«Pr
T k8PC7H14
Gk nC H
V C7H14 7
k8PC7H14
k8PC7H14
k6PO,
k8PC7H1. k7PO,
k8PC7H14
k5Po,
k8PC7H14
k6Po,
k8PC7H1
k7Po,
-\2
| | k8PC7Hi4
dA
4
ki3KiPiK6P2
vnC H
V C7H14 7
,k8PC7Hi4
+
+
(i+KiPi + VK^+K3P3 + K4P4 + K5P5 + K6P2 J2
ki4K3P3K6P2
ki5K4P4K6P2
-^ 33-^- + -i5--^-
(i+KiPi + Vk^+K3P3+K4P4 + K5P5 + K6P2 J2 (i+KiPi+VKP2+K3P3+K4P4 + K5P5+K6P2 J2
2
>
<
2
<
:
2
+
+
+
+
+
k
9
k
9
d
P1 = PC7H14 , P2 = Po2 , P3 = PC7H12 , P4 = PC7H10 , P5 = PC7H8 , P6 = H2O; A1, A2, A3 and A4 according to indices C7Hi2, C7Hi0, C7H8 va CO2 emissions.
Partial pressures of the components are calculated using the following formula:
n
P = —1—P 1 En,
Pi - i- partial pressures of the component, atm; P - total system pressure (1 atm). The material balance for both types of reactors is as follows:
nC7HH = nCyH14 " (A1nC?HM " A2nC7H14 " A3nCyH14 " A4nC7HH )/100
nH2O = (A1nC?H14 + A2nC?H14 + A3nC?H14 + 5A4nC6H12)/100
nO2 = nO2 -(lAlnC7H14 -lA2nC7H14 - 10-5A4nC7H14)/100
= AnL /100
nc7H12 " A1nc7H14
nC7H10 = A2nC7H14/100 0
= . /100
nC7H8 = A3nC H14
nCO2 = 7A4nCyH14/100
The process was studied for both types of methylcyclohexane using a personal computer. reactors on the basis of given kinetic models in Fig.1 shows the results of studies in the
various technological modes: T = 320-400°C, range of molar ratios of methylcyclohexane and
space velocity V = 1000-3000 h-1, oxygen partial oxygen = (0.00902 - 0.02706) mol/h - (0.01129 -
pressure atm, and partial pressure of
0.03386) mol/h at 380 °C.
Fig. 1. Dependence of the conversion (Xs, Xq) rate and selectivity (Ss, Sq) of the process on the contact time in flow reactors of ideal mixing and tubular (packed-bed) reactor.
As can be seen from Fig 1, the coefficient of utilization of the reaction volume at all degrees of conversion in the tubular ( packed-bed) reactor is greater than in the mixing reactor. The increase in the degree of conversion from 35% to 50% is the ratio of the volumes of the studied reactors Vmix./Vcomp. It rises from 1.23 to 1.45. Similar results are observed in other technological modes. Also, studies based on the kinetic model showed
that the selectivity for the target product in the mixing reactor was less than in the tubular (packed-bed) reactor with increasing conversion.
Thus, studies based on kinetic models showed that the optimal reactor for the process of oxidative dehydrogenation of methylcyclohexane into methylcyclohexdiene was an ideal c tubular (packed-bed) type reactor.
Theoretical process optimization
A theoretical optimization of the process based on the kinetic equation was carried out to ensure the maximum productivity of the catalyst in terms of the target product -methylcyclohexadiene. As a result of theoretical
optimization, the optimal technological regimes of the process were determined. Methylcyclohexadiene performance can be determined by the following formula:
( с л
T n0 ft Gkit
1,ПС6Н11СИз, no
V C6H7CH3 J
where qCfiH7CH, - performance of the catalyst time, (grcat *h)/md; 9 - molar ratio of
methylcyclohexane into oxygen, non-uniform
-6H7CH3
for methylcyclohexadiene, gr/(h* grcat); T -
.0
Qcat measurement; П1 - initial cost of reactor temperature, °С; -o--contact methylcyclohexane, mol/h.
CH,,CH,
Based on the developed kinetic model mode parameters that provide the maximum [4], the target function (optimization criteria) efficiency of the catalyst: can be expressed as follows to determine the
max(qcH7Œ,) = f
( С Л
т „o û Gkit
T,nC6HnCH3, no
C6H11CH3 J
According to the technological into account when determining the parameters: conditions, the following limitations were taken
3200C < T < 4000C 70 < 0Gkat < 200
n C6H11CH3
0.01 < n° m < 0.05
C6H11CH3
0.5 < 0 < 2.0
The problem was solved on the basis of the performance of the catalyst based on the the kinetic model of the process and the material kinetic model was calculated for each option using balance. In considering the existing limitations, the following formula:
^Xn^ U flU u /"nu
q _ _C6H7CH3 C6H7CH3
G
cat
where мс H Œ - molecular weight of methylcyclohexadiene.
The problem was solved on a personal of these conditions, the productivity of the
computer using the "Matlab" software system catalyst was qCHCH =0.250 gr/(grcat h), the
[6] by using simplex search method of Nelder- degree of conve^rsion of methylcyclohexane
Mead [7] and the following results were x=5°%, the yield of methylcyclohexane was
obtained: T=380°C; nC6HlCH3 = 0.024mol/h; 15%.
P
9=0.9; 0 cat = 150(grcat h)/mol. As a result
Compilation of the complete mathematical model of the process
Therefore, the yields of products obtained by theoretical optimization based only on kinetic models are conditionally maximized. At this stage, a complete mathematical model of the process was developed by adding heat equations and pressure drop equations to the kinetic equations to obtain a more accurate
description of the flow distribution.
The best approximation process for an ideal tubular (packed-bed) reactor is to be carried out in a stationary bed catalyst reactor, and its mathematical model can be summarized as follows:
de fe. +1 aq 1 q ^ +j ( )= 0
E |aR2 r sr J ai j=t 1j jV 1 '
aE f^ +1 — V qCp aT + ^Qjrj(Ci,T) = 0 E |sr2 r aR J p ai j-rj
where DE - effective diffusion coefficient; R -coordinate in the radial direction; g - mass density; - the stoichiometric coefficient of i-substance in vij - j reaction; q - mass velocity; rj - j-speed of reaction; aE - effective thermal conductivity of the layer; Qj - thermal effect of j-reaction; T - reactor temperature; l - the length of the reactor; Cp - heat capacities of substances.
To achieve the mass velocity of methylcyclohexane at the outlet of the reactor Q = 2000 kg/h, the value of the catalyst volume was determined based on the maximum
C \
gr
productivity
q = 0.250-
grath
using the following formula:
of the catalyst
Vcat = -
Q
<9.5 m3
q ■ Pcat
where pcat - catalyst density, pcat= 850 kg/m .
The height of the specified catalyst mass H = 4.4 m and it can be placed in a cylindrical reactor with a diameter of D =1.7 m in the form of a seamless layer. The heat balance of the process is written as follows according to the material balance:
EQd =£Qt
where Qd - incoming heat, Qt - heat that leaves. In practical calculations, it is necessary to take (Qit) into account the heat losses:
EQd =EQt +EQ
The thermal effects of reactions play a
very important role in chemical processes. The heat included here is as follows:
EQd =Qi+Q2 + Q3
Qi - temperatures of substances entering the apparatus, Q2 - heat entering from the outside, Q3 - thermal effects of physical or chemical transformations.
The heat balance equation includes the thermal effects of chemical reactions, as well as heat losses to the environment through the reactor walls. Accepting some simplifications, the above equation which takes into account temperature changes in general, can be expressed as follows:
dT dG„,
m
Z
Rj
j=i
a(T - Tx )
Z^C* ZZniC
i=1
i=1
where rj - rates of formation of reaction products, mol/(kgcat-h); AHRj - j- thermal
effect of the reaction, kDj/mol; Cpi (i = 1,k) -
values of heat capacities of process components corresponding to the corresponding indices, Dj/(mol-K); a - thermal conductivity, Dj/(K-kgcat-h); Tx - ambient temperature, K; T -
temperature of the gas mixture, K; Gcat -amount of catalyst, kg; m - total number of reactions; ni - mole velocity of the i-component, mol/h.
The below-cited are thermochemical equations for the oxidative dehydrogenation of methylcyclohexadiene for standard conditions [8]:
C7H14 + 0.502 C7H14 + 0.502 C7H14 +1.50-
—^ C7H12 + H2O
^ C7H10 + 2H0
C7H14 +10.50 —
C7H12+1002 ——1 C7H10+9.502 —k
->c7H + 3H0
->7C0 + 7H0
->7C0 + 6H0
^ 7C02 + 5H20
AH = -12.851 kcal/mol AH = -70.533 kcal/mol AH = -124.421 kcal/mol AH = -1030.994 kcal/mol AH = -1013.123kcal/mol AH = -955.442 kcal/mol
To solve the heat balance equation, it is dependence of the thermal effect of each necessary to determine the temperature reaction [8,9]:
1) for isobaric heat capacity of components:
Cpi = ai + biT + ciT2 + diT3
2) for changes in the isobar heat capacity of the system:
r
ACpj = Aa+ AbT + j + AdjT3 =
\
Z viapro -Z Viainit
+
+
ar -Z ViaS
V i i y
Z vibpro - Z Vibr1 ^T + fz vicpro - Z Vicr1 V +fz vidpro - Z Vidi
i y
Vi
-Z vi
i y V i
\
mit
Vidi -Z V
V i i y
T3
3) standard heat of j-reaction:
AH298j =
ZviAH™ -ZviAH
mit
298
V i
4) j-reaction temperature change:
AHj = JACpjdT + AH298 = Aaj(T -298)+ ^(t2 -2982)+ -Ci(t3 -2983)+
298
+ ^(t4 -2984)+ AH29,
4
and in view of the values of standard formation temperatures AH oj and all reactions involved in the process by means of empirical coefficients of heat capacity, the temperature dependence of the AH Rj thermal effects was determined in
accordance with their stoichiometric scheme where: Vj va Vj - stoichiometric coefficients of the initial substances and the i-component of the
reaction products; AHHf, AHp™ - standard formation temperatures of the initial substances and the i-component of the reaction products; AH Rj - thermal effect of j-reaction; AH 0j -
standard thermal effect of j-reaction. If we express the mass of the catalyst by its density, diameter and length of the reactor, the heat balance equation can be expressed as follows:
m
£ rAHj
N0 dT_ £ j Rj a(T - Tx )
dl n n
Pk ■ £ niCpj £ niCpj
4 i=i j=1
where A; - i-output of the product,%; pcat -density of the catalyst; l - length of the reactor; D-diameter of the reactor; rj- j-speed of reaction.
The passage of reagents through flow-type reactors is accompanied by pressure losses. The process is carried out at atmospheric pressure. Although the pressure drop is not so great here, the pressure drop must be taken into account in order to obtain a more accurate and precise distribution of the productivity of the
dP f150 .
— = -|-+1.
dl I Re
reaction products along the length of the reactor.
The calculation equation proposed by Ergun is used [5,10] to calculate the pressure drop for heterogeneous catalytic processes which well describes the experimental estimates of various authors on the resistance in the catalyst layer and can be used to calculate the pressure loss along the reactor length of the studied process:
^ PgasUQ 0 - s)
J dpgs3 '
where Re - is the Reynolds criterion,
D - diameter of the reactor, m; density, kg/m3; g - gravity, m/s2;
dpPgasU0 - *);
as - gas , u0 - linear
diameter of
speed, m/s; dp - equivalent
particles, m ; s - porosity; p - viscosity of gas, kg/m •s); l - the length of the reactor, m.
Kinetic equations of the process of oxidative dehydrogenation of
methylcyclohexane to methylcyclohexadiene, equations taking into account the heat balance and pressure drop form a complete mathematical model of the process:
ni dAl _ „1 „2
nD2 Pcat 4 dl
ni dA2
nD2 Pcat 4 dl
ni dA3
nD2 Pcat 4 dl
ni dA4
rC7H12 rCO2
= r1 - r3
C H CO
r<C7H8
nD2 dl
Pcat ' 4
12 3
= r1 + r2 + r3
2 J1 rCO2 ^ rCO2 ^ rCO2
dP f 150 , , „pgasu2i1 - e)
- +1.751'-
dl ^ Re J dpge3
m
V rAHRj , x N0 dT_ V rj a(T - t )
P ' ^ dl V n;Cpi V n;Cpi
i=1 i=1
<
where r<^, r202, r^ - rates of carbon dioxide methylcyclohexadiene (C7H10) ; r1 H, r<î. H . formation from methylcyclohexane (C7Hi4), r£ H - formation rates of methylcyclohexane^
methylcyclohexene (C7H12) and , , , , . ,
J J v / methylcyclohexadiene and toluene.
Conclusion
Thus, based on the above results, we can conclude that the optimal reactor for the process of selective oxidative dehydrogenation of methylcyclohexane into methylcyclohexadiene is an ideal tubular (packed-bed) type reactor, and it is more expedient to carry out the process in a stationary catalytic reactor. The adequacy of the developed mathematical model was tested on an experimental device and its accuracy was proved. Studies on a mathematical model showed a relatively small pressure drop (from 1 atm to 0.9 atm). This does not affect the overall
process and there is no need to increase the linear flow rate. There is also some increase in temperature along the length of the reactor (from 380°C to 385°C). Allowing for the small temperature difference, there is no need to use expensive isothermal reactors, and an adiabatic type reactor can be used for the process under study. The results obtained can be used in the development of the process of the oxidative dehydrogenation of methylcyclohexane into methylcyclohexadiene on a modified active metal seolite catalyst.
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2. Aliev A.M., Shabanova Z.A., Karimov A.I., Najaf-Guliyev U.M. Studies of the catalytic activity of the modified zeolite in the oxidative dehydrogenation of methylcyclohexane. 1st
Int. Turkic World Conf. Chem. Sci. Technologies. Sarayevo. 2015, p. 320.
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4. Aliyev A.M., Shabanova Z.A., Kerimov A.I., Bahmanov M.F., Aliyev F.V., Najaf-Guliyev U.M. Use of metal-zeolites as a catalyst in reaction of oxidative dehydrogenation of naphthenes. Azerbaijan Chemical Journal. Azerbaijan, Baku 2016, no. 3, pp. 63-74.
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6. Yeo, Yeong Koo, Chemical engineering computation with Matlab, Taylor & Francis, CRC Press, 2017, 608 p.
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MODiFiKASiYA OLUNMU§ SEOLiTKATALiZATORU ÜZdRINDd METiLTSiKLOHEKSANINMETiLTSiKLOHEKSADiENd OKSiDLd§DiRiCi DEHiDROGENLd$MdSiPROSESiNiN OPTiMAL LAYiHdLd§DiRiLMdSiNiNELMi
OSASLARI
d.i. Krimov
AMEA-nm akad. M.Nagiyev adina Kataliz vd Qeyri-üzvi Kimya institutu AZ1143, Baki, H.Cavid pr., 113; e-mail: kerimov.alihala amail.ru
Xülasa: Modifikasiya olunmu§ aktiv metalseolit katalizatoru üzdrindd metiltsikloheksanin metiltsikloheksadiend selektiv oksidld^dirici dehidrogenld§dirmd prosesin kinetik modeli dsasinda reaktor tipinin segimi vd ndzdri optimalla§dirilmasi aparilmi§dir. Müdyydn olunmu§dur ki, baxilan prosesi ideal tipli reaktorda aparilmasi daha mdqsddduygundur. Prosesin ndzdri optimalla§dirilmasi ndticdsindd optimal texnoloji rejimldri tdyin olunmu§dur. Reaktor elementinin verilmi§ mdhsuldarliga görd optimal konstruktiv ölgüldri hesablanmi§dir. istilik effektldrini vd tdzyiq dü^güsünü ndzdrd alaraq prosesin tam riyazi modeli tdrtib olunmu§dur. Agar sözlw. metiltsikloheksan, metiltsikloheksadien, oksidld^dirici dehidrogenld§md, klinoptilolit, reaktor segimi, ndzdri optimalla§dirma, riyazi model.
НА УЧНЫЕ ОСНОВЫ ОПТИМАЛЬНОГО ПРОЕКТИРОВАНИЯ ОКИСЛИТЕЛЬНОГО ДЕГИДРИРОВАНИЯ МЕТИЦИКЛОГЕКСАНА В МЕТИЛЦИКЛОГЕКСАДИЕН НА МОДИФИЦИРОВАННОМ ЦЕОЛИТНОМ КАТАЛИЗА ТОРЕ
А.И. Керимов
Институт катализа и неорганической химии им. акад. М.Нагиева
Национальной АН Азербайджана AZ1143 Баку, пр.Г.Джавида, 113; e-mail: kerimov.alihalaamail.ru
Аннотация: На основе кинетической модели процесса селективного окислительного дегидрирования метициклогексана в метилциклогексадиен на модифицированном цеолитном катализаторе осуществлен выбор оптимального типа реактора и теоретическая оптимизация данного процесса. Было выявлено, что реактор идеального вытеснения является наиболее целесообразным для ведения рассматриваемого процесса. В результате теоретической оптимизации были определены оптимальные технологические режимы. Исходя из заданной производительности, рассчитаны оптимальные конструктивные размеры реакторного элемента. С учетом тепловых эффектов и перепада давления составлена полная математическая модель процесса.
Ключевые слова: метилциклогексан, метилциклогексадиен, окислительное дегидрирование, клиноптилолит, выбор реактора, теоретическая оптимизация, математическая модель.
