2371
Ind. Eng. Chem. Res. 1996,34, 2371-2378
Kinetics of the Hydrogenation of Diethyl Oxalate to Ethylene Glycol Gen-hui Xu,* Yan-chun Li, Zhen-hua Li, and Hai-Jing Wang Department of Chemical Engineering, Tianjin University, Tianjin, 300072, People's Republic of China
The reaction kinetics of the catalytic hydrogenation of diethyl oxalate to ethylene glycol in the vapor phase over a copper-base catalyst were studied. The experiments were carried out in a continuous flow microreactor. The experimental work was based on the following consecutive
-
-
+
+
+
reaction scheme: CzH&OOCOOCzH5 4H2 HOCHzCHzOH 2CzH50H; HOCHzCHzOH H2 C2H50H HzO. Fourteen competing kinetic models obtained from the possible mechanisms have been proposed for the above scheme. By fitting the experimental data to each model, the following rate equations were found to best fit the data: r1 = [K~KoxKH(Po*H - PEGPE2/KPPH3Y(1 KOX~OX + + KEGPEG K ~ E )r2~=; k&E&HpEGPH/(1 + KOWOX KEGPEG &pd3.In the model from the above rate equations were obtained the hydrogenation reaction of diethyl oxalate follows the Langmuir-Hinshelwood mechanism in which hydrogen adsorbs dissociatively. The surface reaction step was rate-limiting for both the main and side reaction.
d=
+
+
+
+
JG
Introduction Ethylene glycol is a very important chemical, and a t present the use of ethylene oxidation to produce ethylene glycol is the universal industrial approach. However, as crude oil resources shrink, substitutes for petroleum feedstocks will be of great significance. Among others, the indirect synthesis of ethylene glycol from syngas is of great interest. Several patents for indirect synthesis to ethylene glycol have been published in recent years (Union Carbide Corp. 1974; Bartley, et.al., 1987; Eur. Chem. News, 1981; Ube Industries Ltd., 1989; Aqullo et al., 1983). The present work is based on o u r previous studies of catalysts for carbon monoxide coupling reaction and hydrogenation of diethyl oxalate. The present work investigates the two-step synthesis (indirect synthesis) to ethylene glycol. The reaction equations are
+ 2ROH + (1/2)0, - (COOR), + H,O (COOR), + 4H2 - HOCH2CH20H+ 2ROH 2CO
(1) (2)
where R- and ROH are the alkyl and alkyl alcohol, respectively. The first step is the carbon monoxide coupling reaction t o diethyl oxalate; the second step is diethyl oxalate hydrogenation to ethylene glycol. The total reaction equation is 2CO
+ 4H, + (1/2)0, - HOCH,CH,OH + H,O
+ 2H, -
C,H,COOCH,OH
+ C,H,OH
Table 1. Equilibrium Constants of Reactions 4-7 at Three Temperatures temmrature ("C1 Kpi KP2 KP Kp3
25
200
94.9 12.8 1.21 103 3.21 x 10l6
1.52 0.26 0.4 6.06 x 1O1O
C,H,CO,CH,OH
(4)
* Author to whom correspondence should be addressed. 0888-588519512634-2371$09.0010
250
0.77 0.14 0.13 7.09
109
+ 2H, -
+ C,H,OH + H, - C,H,OH + H 2 0
HOCH,CH,OH
(5)
HOCH,CH,OH (6) Thermodynamic calculation data of the above three reactions are listed in Table 1. Reactions 4 and 5 are thermodynamically reversible reactions, while reaction 6 is the irreversible side reaction. Assuming that the overhydrogenationreaction 6 is avoided, the dependence of the equilibrium yield of ethylene glycol (Y) on the reaction pressure (P)and the hydrogedester ratio (Hd ester) is shown in Figure 1. For pressures above 1.0 MPa, with Hdester above 30/1, the formation of ethylene glycol was favored. For further discussions the two reactions 4 and 5 were combined to one reaction and the experimental planning was based on the following consecutive reaction scheme: C,H,COOCOOC2H,
(3)
Equation 3 is the selective oxidation to ethylene glycol from syngas. Our studies of step 1, the CO coupling reaction and kinetics, have been already presented (Xu et al., 1990, 1991; Li et al., 1991; Chen et al., 1993). This paper reports kinetics of the hydrogenation of diethyl oxalate t o ethylene glycol (step 2). The experiments were carried out in a flow microreactor. Previous investigations showed (Poppeisdorf, 1982; Bartley, 1986) the reaction for hydrogenation of diethyl oxalate to ethylene glycol, as shown by: C,H,COOCOOC,H,
+
+ 4H, -
+ 2C,H,OH + H, - C,H,OH + H,O
HOCH,CH,OH
(7)
(8) Fourteen competing kinetic models have been proposed for the above scheme from the possible mechanisms. The best kinetic model was determined by the nonlinear optimum methods, i.e., the damped least square method (Jacobs, 1977; Bevington, 1969). HOCH,CH,OH
Experiment The kinetic experiments of diethyl oxalate hydrogenation to ethylene glycol were carried out on the Cu/SiO2 catalyst under the steady-state condition of activity of catalyst. The CUI Si02 catalyst was prepared by the precipitation method. Cupric nitrate was dissolved in deionized water and ammonia water was slowly added 0 1995 American Chemical Society
2372 Ind. Eng. Chem. Res., Vol. 34, No. 7, 1995
A
1.00
B
1.00
*
0.50
*
0.50
0.00 0.00
1.00
2.00
0.00. 0.00
3.00
3.00
2.00
1.00
P(MPa)
P(MPa)
Figure 1. Relation between Y,pressure, and Hdester. at (A) 250 "C and (B)250 "C. 1,Hdester
= 4:l;
2, Hdester = 1O:l; 3, Hdester =
20:l; 4, Hdester = 30:l.
I
Di
09
08
02 01
0
Figure 2. The experimental flow diagram. 1, H2 tank; 2, stable pressure valve; 3, pressure gauge; 4, flow-transmit sensor; 5, flowregulation valve; 6, mass flow controller; 7, unidirectional valve; 8, mixer; 9, reactor; 10, electric heater; 11,condensate separator (pressure); 12, sampler; 13, stable pressure valve; 14, condensate separator (atmosphere);15, flowmeter; 16, wet flowmeter; 17, SY02 micropump; 18, thermoregulator; 19, voltage adjustor; 20, thermocouple; 21, thermocouple.
t o the solution until the pH reached 9 f 0.5, resulting in the indigo-blue cuprammonia complex solution while stirring and heating; then silica gel solution was added for subsequent formation of gelatinous precipitation. The precipitate was washed and dried a t 120 "C, and
4
o m a D m 4 3 3
o
m
"
Mao(g&ndJ
)--
Figure 3. Experimental results of rate isothermal curves at different temperature (T),pressure (PI,and Hdester ratio (H2:e) reaction conditions. (A) T = 250 "C; P = 1.0 MPa; H2:e = 30:l. (B) T = 220 "C; P = 2.5 MPa; H2:e = 1OO:l. (C) T = 235 "C; P = 2.5 MPa; H2:e = 1OO:l. (D)T = 220 "C; P = 2.0 MPa; H2:e = 50:l.
the catalytic solid of copper-base CdSiOz was obtained. The compositions of the catalysts are expressed in weight percent, the CdSiOz catalyst having Cu loadings in the range 20-30 wt %. Details of the preparation
Table 2. Emerimental and Calculated Kinetic Results reaction condition temp 250 "C
WIFAO X
xc
EX Y Yc
44.31 0.3245 0.3279 1.05 0.0862 0.0787
EY
-8.81
Pox PH
0.0268 1.9311 0.0095 0.0292 0.0034 6.075 2.389
PEG PE pw rl x
lo3
r2 x lo3
59.09 0.4290 0.4068 -5.17 0.1321 0.1059 9.83 0.0228 1.9206 0.0118 0.0395 0.0053 5.197 2.453
press. 2.0 MPa 88.63 0.5469 0.5303 -3.04 0.1988 0.1604 -9.32 0.0182 1.9081 0.0140 0.0518 0.0080 3.685 2.362
118.17 0.6340 0.6235 1.66 0.2618 0.2131 8.60 0.0147 1.8983 0.0150 0.0615 0.0105 2.522 2.089
Hdester 5011 151.94 0.7100 0.7047 -0.75 0.3303 0.2708 -8.01 0.0117 1.8891 0.0153 0.0706 0.0133 1.660 1.732
212.71 0.7940 0.8037 1.22 0.4244 0.3645 4.11 0.0083 1.8781 0.0150 0.0815 0.0172 1.510 1.593
temp 235 "C 45.44 0.2747 0.2538 -7.61 0.0556 0.0497 -1.06 0.0192 1.9575 0.0058
0.0160 0.0015 4.525 1.070
60.59 0.3161 0.3207 1.46 0.0662 0.0660 -3.00 0.0182 1.9549 0.0066 0.0185 0.0018 3.954 1.046
press. 2.0MPa 83.90 0.3895 0.4123 5.85 0.0868 0.0911 4.95 0.0162 1.9504 0.0081
0.0230 0.0023 3.243 1.011
136.33 0.5553 0.5712 2.86 0.1450 0.1457 0.48 0.0119 1.9398 0.0110 0.0335 0.0039 2.239 0.942
Hdester 75/1 218.13 0.6973 0.7324 5.03 0.2149 0.2253 4.84 0.0081 1.9301 0.0129 0.0342 0.0058 1.618 0.841
363.56 0.8358 0.8803 5.32 0.3207 0.3503 9.23 0.0044 1.9195 0.0139 0.0536 0.0086 1.102 0.577
,a
0.9
0.7
0.8
0.6
0.7 0.5
0.6 0.5
g 0.4
0
0.3
0.2
0.2
0.1
0. i 1
0
0.005
0.01
0.02
0.015
0
0.005
0
0.01
0.02
0.015
pm
0.025
0.03
POX
(B)1. tB) 2. Figure 4. Kinetic experimental results. (A) Experimental results of reaction rates (r1, r2) versus space velocity (WIFAO):(A)l. at 220 "C, 2.0 MPa, 50:l; (A)2. at 235 "C,2.5 MPa, 1OO:l. (B)Comparison of experimental and calculated (using mechanism 3) reaction conversion (X,Y) versus reactant partial pressure (pox)data: (B)1. at 235 "C, 2.5 MPa, 1OO:l;(B)2. at 250 "C, 1.0 MPa, 30:l.
A
*
2.50
4.00
L
0.00
---
1
2.00
4
1.50 1.oo
0.50
0.0019
0.0020
0.0021
0.0022
'
II -1.00 J I I I I 1 I 0.0019 0.0020
I
I
I' I
0.0021
'
III
'-
0.0022
1/T( 1/K)
Figure 5. Relations between the rate constant and temperature: (A) relation between k1 and T; (B) relation between k2 and T.
method of the catalyst were described in our previous work (Wang, 1992; Li, 1993). Before kinetic reaction experiments the catalyst must be activated under hydrogen atmosphere a t 350 "C, 0.35MPa, and Hz flow rate about 0.2-0.15 L/(min.cm3 (cat)) until the activity of catalyst was stabilized. The experiments were carried out in the flowing system microreactor device. The experimental apparatus and flow scheme are shown in Figure 2. The diethyl oxalate is fed by an SY-02 pressure micropump (17) via the mixer (8),where it is mixed with hydrogen from the H2 tank (1) and then fed to the reactor 9. The reactor was a 600 mm long, 20 mm i.d. stainless tube. The upstream section of tube (about 200 mm) was used as a preheat section. The reaction zone was filled with 40-60 mesh catalyst particles diluted
with quartz chips of the same size. The reaction section was maintained as a constant temperature control zone (about 300 mm). Reaction products were analyzed by means of the SP-3700 GC using a flame ionization detector, with OV-01 column packing. The kinetic experimental conditions were reaction temperature 200-250 "C ,reaction pressures above 1.0 m a , Hdester ratios 3011-100/1, and LHSV (liquid hour space velocity) 0.1875-1.875 h-l. The reaction was performed in a tubular flow microreactor assumed t o behave as a plug flow reactor (PFR). Before the kinetic experiments great attention was paid to elimination of mass-transfer effects and to measurement of the intrinsic reaction rate. The experiments were carried out a t differenti temperatures, pressures, and Hdester ratios, as well as different
2374 Ind. Eng. Chem. Res., Vol. 34, No.7, 1995 Table 3. Kinetic Models no.
characteristic of reaction mechanisms
1
L-H mechanism
2
Rideal mechanism
3
L-H mechanism, hydrogen is disassociation adsorption
4
main reaction is diethyl oxalate adsorption control, side reaction is L-H mechanism
5
main reaction is diethyl oxalate adsorption control, side reaction is Rideal mechanism
6
main reaction is diethyl oxalate adsorption control, side reaction is L-H mechanism
7
main reaction is L-H mechanism, side reaction is desorption control of water
8
main reaction is Rideal mechanism, side reaction is desorption control of water
9
main reaction is L-H mechanism, side reaction is desorption control of water
10
main reaction is ethylene glycol desorption control, side reaction is L-H mechanism
kinetic models
Ind. Eng. Chem. Res., Vol. 34, NO. 7, 1995 2375 Table 3. (Continued) no. characteristic of reaction mechanisms 11 main reaction is ethylene glycol desorption control, side reaction is Rideal mechanism
kinetic models
rl
- r2 =
r2 = 12 main reaction is ethylene glycol desorption control, side reaction is L-H mechanism
rl
- PEJW
k1~P@O~H4/p,2
+ KOXpOX + KEG(K#OXPH4/p:) + K$E ~~KEGPEGPH
(1 + KoXPOX + KEG(K#O$H4/P2)
- r2 =
k1
+ KWPW)2
+ KE?E
~ p @ o ~ Hi24 / ~PEJKP)
+ KOXPOX + &+ KEC(K#OXPH4/PE2)
+ KOXpOX + &+ KEG(K#OXPH4/P2) r1
14 main reaction is desorption control of ethylene glycol, side reaction is desorption control of water, H2 is disassociation adsorption
r1
- r2 =
- r2 =
Y = mol of H,O/mol of (C,H5COO),,, (the feed is without water) (9b) The rates rl and r2 were obtained by differentiation of the isothermal curves, i.e., r1= dX/d(W/FAo),rz = dY/ ~(WIFAO), by means of curve fitting calculated results, at certain (WIFAO), respectively. Examples are listed in Table 2 and shown in Figure 4.
Results and Discussion Kinetics Models Estimation. The above mentioned eqs 7 and 8 were consecutive reactions schemes. The reaction processes according to hydrogenation of ester radical and hydrogenolysis mechanisms are shown below:
+
00
II II
II
00
II II
+
HZ
(10)
0
II
C~HSOCCH~OH
(1 la)
0
II
C~HSOCCH~OH H2 O=CHCHzOH
HOCH2CHzOH
+
C~HSOCCH CzH5OH
+ + -
CzH5OCCH
0
-
CHCH2OH
H2
+
H2
-
+ C~HSOH
HOCHZCH~OH CzH50H
+ H20
+ K$E
+ KWK
