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Numerical Investigation of Semibatch Processes for Hydrogenation of Diene-Based Polymers Qinmin Pan† and Garry L. Rempel* Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada
Dynamic behavior of semibatch processes were numerically investigated for catalytic hydrogenation of diene polymers/copolymers, including 1,4-polybutadiene (PB), diblock (SB) and triblock (SBS) copolymers of styrene and butadiene, and nitrile-butadiene rubber (NBR). Generalized models of the kinetic mechanism for homogeneous catalytic hydrogenation and of coupling behaviors between kinetics and mass transfer were developed for semibatch processes. The sensitivity of various kinetic parameters and the effects of operation conditions on the hydrogenation processes were analyzed, and the evolution of reaction trajectories in the semibatch hydrogenation processes was studied. It is proposed that the coupling behavior between the catalytic hydrogenation and mass transfer was completely determined by the ability of the catalyst in activating hydrogen, carbon-carbon double bond loading level, and the relative capacity of reaction to mass transfer. Three dimensionless parameters were derived to characterize these aspects. An optimal operation surface composed of the proposed three dimensionless parameters was constructed. Further research directions are suggested. Introduction Catalytic hydrogenation of unsaturated polymers is a postpolymerization process used to alter and optimize the chemical and physical properties of the parent polymers. With the availability of a large number of unsaturated polymers of differing microstructures, the selective reduction of carbon-carbon double bonds offers a way of producing a wide variety of specialty polymers. By reducing the unsaturation level of the polymer, the physical properties of the polymer such as tensile strength, elongation, thermal stability, light stability, and solvent resistance may be optimized. For example, the removal of the CdC unsaturation in PB provides a tough semicrystalline polymer similar to linear polyethylene or an elastomer like poly(ethylene-co-butylene), etc., depending on the relative levels of units with 1,2 or 1,4 structure. Hydrogenation of an SBS triblock copolymer with a moderate amount of 1,2-addition units in the center block yields a copolymer with a poly(ethylene-co-butylene) center segment. This modified polymer has greatly increased thermal and oxidative stability, together with processability and serviceability at higher temperature, by virtue of its poly(ethlyeneco-butylene) center block. The catalytic hydrogenation of acrylonitrile-butadiene copolymer is an especially important commercial example, resulting in its tougher and more stable derivative, hydrogenated nitrilebutadiene rubber, which has been widely used in the automobile industry. The catalytic hydrogenation of diene-base polymers actually has become a potential method for the improvement of polymer properties and the production of new specialty polymers. Hydrogenation of diene polymers is carried out preferably via homogeneous catalysis and generally realized * E-mail:
[email protected]. Tel.: +1-519-8884567, ext. 2702. Fax: +1-519-7464979. † On leave from the Department of Chemical Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China
in a semibatch process accompanied with hydrogen transfer from the gas to the liquid phase. Effective catalysts and the use of efficient reactors are the key among the hydrogenation techniques. Knowledge of kinetics and dynamic performance is essential in realizing an efficient hydrogenation process. Much progress in this field has been made during the past couple of decades. Several catalysts have been exploited successfully for the hydrogenation in organic solvents, including palladium,1 rhodium,2-4 ruthenium,5 and osmium6 complexes. Ru- and Os-based catalytic systems5,7 are receiving increasing attention due to their effectiveness and much lower cost than the Rh-based systems. Kinetic investigations under a wide range of experimental conditions have been reported for the Rh-based catalytic hydrogenation.2-4,8-10 The hydrogenation mechanism involves the activation and transfer of molecular hydrogen by transition-metal complexes, the saturation reaction of olefin within the parent polymers and some possible side coordination reactions. The catalytic hydrogenation of the diene polymers in bulk was also reported by Gilliom.11 Perhaps due to the difficulty resulting from mass transfer and heat transfer it did not receive much attention. Besides the hydrogenation in organic solvents or bulk, a new concept based on aqueous-phase hydrogenation using water-soluble rhodium catalysts also has been proposed and attempted.12 It highlighted a potential way to simplify the operation and reduce the production cost by integrating the polymerization and the hydrogenation modification into one single process. A comprehensive review of the research in this field has been reported by McManus and Rempel.13 Considerable effort has been made to develop effective catalysts and to understand hydrogenation kinetics, but little has been done to exploit highly efficient reactors and to reveal the dynamic coupling performance between the reactions and transport behaviors of the adopted reactors. The main concern in this field is currently how to increase the operation concentration
10.1021/ie9904347 CCC: $19.00 © 2000 American Chemical Society Published on Web 01/15/2000
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of polymers, decrease the demanded quantity of catalyst and organic solvents, and optimize the operation conditions to lower the production cost. All these depend on a detailed understanding of the hydrogenation process. The objective of this paper is to investigate the coupling behaviors between the reaction and hydrogen transfer as a continuation of previous studies carried out in our laboratory. Development of a Coupling Model Though the hydrogenation operation concerned here is referred to as “homogeneous”, it is accompanied with hydrogen transfer from the gas into the liquid phase. A distinguishing characteristic of this homogeneous hydrogenation operation is that the process is affected by the gas-liquid mass transfer behavior. A model characterizing the coupling behavior between the reaction and hydrogen transfer is developed here to investigate the coupling behavior. The present and previous hydrogenation experiments in our laboratory were carried out in a semibatch manner in order to simulate the industrial operation. A model apparatus was described in Mohammadi and Rempel’s paper.14 It is operated under isothermal and isobaric conditions with constant agitator speed in the liquid phase. Hydrogen flows continuously into the reactor to keep the pressure constant. Both CdC and hydrogen concentrations vary with time t. It is assumed here that the hydrogen transfer from the gas into the liquid phase could be described by Whitman’s film model,15 and the extent of the reaction in the liquid film was negligible. The changes in density, diffusivity, and hydrodynamic state of reaction liquid were considered negligible during the hydrogenation process. Thus, the following nonlinear ordinary differential equation set represents the hydrogenation processes:
d[CdC] ) -RH dt
(1a)
d[H2] ) KLa([H2]e - [H2]) - RH dt
(1b)
with the initial conditions
[H2](0) ) [H2]e
(2a)
[CdC](0) ) [CdC]0
(2b)
[H2](0) ) 0
(3a)
[CdC](0) ) [CdC]0
(3b)
or
where [H2]e is the hydrogen concentration in the liquidphase equilibrated with the hydrogen pressure in the gas phase. RH in eq 1 is the intrinsic hydrogenation reaction rate. The expression of RH depends on the kinetic mechanism, which has been experimentally investigated for the hydrogenation of NBR, PB, SB, and SBS in our laboratory under a wide range of operation conditions. The typical operation conditions are shown in Table 1. The kinetic mechanism for the hydrogenation systems involved can be generalized into the scheme shown in
Figure 1. polymers.
Diagram of hydrogenation mechanism of diene-
Table 1. Typical Conditions for Hydrogenation of PB, SB, SBS, and NBR polymer
[Rh], [PPh3], mM mM
T, K
[H2], mM (PH2, MPa)
solvent
NBR96 NBR87 SBS SB 1,4PB
0.080 1.958 1.99 2.04 1.99
418.2 313.2 338.2 324.2 338.2
101(2.37) 3.142(0.10) 3.90(0.10) 3.17(0.10) 3.90(0.10)
chlorobenzene butanone o-dicholorobenzene toluene o-dicholorobenzene
4.0 0 7.40 0 7.40
Figure 1 by comparing the models proposed by McManus and Rempel,2 Mohammadi and Rempel,8 Guo and Rempel,9 Guo et al.,10 and Parent et al.3 It was proposed that the activation of CdC was the controlling step in all of these models. So the corresponding general expression of RH of Figure 1 is as follows:
RH ) k′K′KK1[H2][Rh][CdC]/{KK1 + K′[PPh3] + KK′[H2][PPh3] + KK1K′[H2] + KK1K5[CN] + KK1K2K′[H2][CN]} (4) Here [H2] and [CdC] are time dependent and are determined by kinetic and mass transfer behaviors. The kinetic parameters are listed in Table 2. KLa in eq 1 is the overall volumetric mass transfer coefficient between the gas and liquid phases. In the present hydrogenation system KLa is actually equal to kLa, the volumetric mass transfer coefficient in the liquid phase, because both mass transfer resistance in the gas phase and at the interface between gas and liquid phases may be considered negligible. kLa is determined by the adopted reactor, reaction fluids, and operation conditions. Parent16 has carried out a preliminary experimental investigation into the mass transfer performance of the reaction fluids in a hydrogenation reactor. If a more detailed correlation of mass transfer performance over a range of operation conditions were available, it could be substituted into eq 1b and the analysis could be extended to the detailed design of hydrogenation reactors for the necessary mass transfer capacity. Here, in this paper, the focus is on understanding the coupling behavior between hydrogenation and mass transfer and obtaining the principles for selection or design of the reactors.
Ind. Eng. Chem. Res., Vol. 39, No. 2, 2000 279 Table 2. Kinetic and Coordination Equilibrium Constants polymer (solvent)
K, (mM)-1
NBR96 (chlorobenzene) NBR87 (butanone) SBS (o-dicholorobenzene) SB (toluene) 1,4PB (o-dicholorobenzene)
∞ ∞ 0.31 1.23 0.60
K1, mM 1.44 0.198 3.13 4.70 4.45
K′, (mM)-1
K2, (mM)-1
K5, (mM)-1
k′, (mM s)-1
E, kJ/mol
10-3
10-2
10-2
1.19 4.23 × 10-4 3.26 × 10-4 4.77 × 10-4 1.28 × 10-3
73.5 87.3 78.8 60.8 98.5
3.41 × 0.276 0.63 0.72 0.59
3.98 × 6.5 × 10-2 0 0 0
2.71 × 0 0 0 0
with the initial conditions
h(0) ) 1
(2a′)
x(0) ) 0
(2b′)
h(0) ) 0
(3a′)
x(0) ) 0
(3b′)
or
where x is the degree of hydrogenation or conversion, h is the dimensionless hydrogen concentration, and θ is dimensionless time, which are defined as
h ) [H2]/[H2]e
(5)
and
θ ) t/τr ) k′K1K′[H2]e[Rh]Tt/{K1 + K1K5[CN] + K′([PPh3]/K + ([PPh3] + K1 + K1K2[CN])[H2]e)} (6) τr is the time constant under pure chemical reaction conditions without mass transfer resistance, i.e.
Figure 2. Comparison of experimental data and model prediction for NBR hydrogenation.
Obviously, there is no special assumption in the model except the application of basic principles and common assumptions. Nevertheless, the test for validation of the model is still performed for all systems involved. The experimental method and operation procedure involved in the present research is essentially the same as that used by Parent et al.3 Figure 2 shows several experimental examples of NBR hydrogenation carried out by different investigators under different temperature, hydrogen pressure, and solvent media. Results for other hydrogenation systems are quite similar but omitted here to conserve space. Considering no adjustable parameter being added artificially in the model, the agreement is acceptable between the predicted values of the model and the experimental data despite some deviation appearing in the system operated under lower temperature.
τr ) {K1 + K1K5[CN] + K′([PPh3]/K + ([PPh3] + K1 + K1K2[CN])[H2]e)}/k′K1K′[H2]e[Rh]T (7) The dimensionless parameters q, b, and R stand for
q ) [CdC]0/[H2]e
b ) {(K1K + KK′[PPh3] + KK1K2K′[CN])[H2]e}/ {KK1 + K′[PPh3] + KK1K5[CN]} (9) R ) k′K1K′[Rh][CdC]0/{kLa(K1 + K1K5[CN] + K′([PPh3]/K + ([PPh3] + K1 + K1K2[CN])[H2]e))} (10) All of these three parameters possess exact physical meaning. The meaning of q is explicit: the ratio of the initial concentration of CdC to be hydrogenated to the hydrogen concentration equilibrated with the hydrogen pressure in the gas phase, standing for the loading level of CdC. The formation of eq 9 can be transformed to
b)
Analysis of Characteristic Parameters To reveal the key factors controlling the model and in turn the coupling behavior, the dimensionless transformation is performed according to eqs 1 and 2, and the following expressions are obtained:
dx (1 + b)h(1 - x) ) dθ 1 + bh
(1a′)
(1 + b)h(1 - x) 1 dh 1 ) (1 - h) q dθ R 1 + bh
(1b′)
(8)
[A] + [B] + [C] [A0] + [B0] + [C0]
|
(9a)
[H2])[H2]e
i.e., b represents the ability of catalyst to activate molecular hydrogen. And R is the ratio of the maximal consumption rate of hydrogen in the hydrogenation reaction,
RHmax ) k′K′KK1[H2]e[Rh]T[CdC]0/{KK1 + K′[PPh3] + KK′[H2]e[PPh3] + KK1K′[H2]e + KK1K5[CN] + KK1K2K′[H2]e[CN]} (11)
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Table 3. Examples of R, q, and b under General Operation Conditions polymer (solvent)
[Rh], mM
[PPh3], mM
T, K
[H2], mM (PH2, MPa)
q
b
R
τr
NBR96 (chlorobenzene) NBR87 (butanone) SBS (o-dicholorobenzene) SB (toluene) 1,4PB (o-dicholorobenzene)
0.080 1.958 1.99 2.04 1.99
4.0 0 7.40 0 7.40
418.2 313.2 338.2 324.2 338.2
101 (2.37) 3.142 (0.10) 3.90 (0.10) 3.17 (0.10) 3.90 (0.10)
2.72 96.4 69.7 140 51
0.65 10.6 1.42 2.3 2.3
0.03 0.019 0.026 0.31 0.11
284 16177 8827 1478 1495
to the maximal physical mass transfer rate of hydrogen from the gas phase into the liquid phase,
RMTmax ) kLa[H2]e
(12)
which depicts the relative capacity of the intrinsic hydrogenation reaction over the mass transfer of the reactor. Therefore, the model, and in turn the coupling behavior, are completely determined by the ability of the catalyst to activate hydrogen, the carbon-carbon double bonds loading level, and the relative capacity between reaction and mass transfer, besides the initial operation conditions. Some typical values of R, q, and b for the systems under consideration are listed in Table 3. Numerical Investigation of the Coupling Behavior Reaction Trajectories and Reaction Rate. Reaction trajectories can be used to describe visually the dynamic characterization of the hydrogenation reaction. They can be obtained by the phase plane analysis method17 based on eq 1. The equilibrium point of eq 1, which is computed from dx/dθ ) 0 and dh/dθ ) 0, is located at x ) 1 and h ) 1 independent of the starting points. The equilibrium point, in fact, is the reaction terminal point desired to be reached rapidly in the hydrogenation process. The eigenvalues of the Jacobian matrix of eq 1 are -1 and -q/R, respectively around the equilibrium point. Because q and R are always positive under real operation conditions, the eigenvalues are constantly negative. This implies that the equilibrium point and the reaction trajectories on the phase plane composed of x and h are steady in the hydrogenation process under isothermal operation. Figures 3 and 4 are some examples of two-dimensional reaction trajectories from θ ) 0-3 starting from two different points (i.e., different initial conditions defined in eqs 2 and 3) and under various operation conditions. This illustrates the effect of three parameters q, R, and b. It can be seen from Figures 3 and 4 that among these three parameters, R is the dominant factor and that low R is preferred for higher hydrogenation degrees. At a given R, the effect of b is not very distinguished, which means the effect of the ability of catalyst to activate hydrogenation is not as important as expected. This suggests the possibility of broadening the search range for new catalysts. However, it should be noted that this result originates from the kinetic assumption where the rate controlling step is not related directly to the activation of molecular hydrogen. The effect of q is dependent on the level of R. When R is small, no significant effect of q exists. But when R is high, higher q will result in a lower hydrogenation degree, and enhancement in mass transfer will be required. This implies that the realization of higher CdC loading level relies on the superior mass transfer behavior of the reactor. For a first-order reaction, the hydrogenation degree x should be about
Figure 3. Effect of b and R on reaction trajectories.
0.95 at θ ) 3 when hydrogen transfer resistance is negligible. Therefore, the deviation of hydrogenation degree at θ ) 3 from 0.95 is also a measure of the effect of mass transfer resistance on the hydrogenation reaction. Optimization of Operation Conditions. Besides understanding the effect of operation parameters on hydrogenation degree and hydrogenation rate, it is often required to determine the profitable operation conditions under a given objective. Considering the problems of as to when the effect of mass transfer resistance is negligible on the hydrogenation and of how to determine the corresponding operation conditions, an objective is defined as follows:
R h H(x)0.95) ) t(x)0.95) 1 RH dt t(x)0.95) 0 real conditions ) 0.9 (13a) t(x)0.95) 1 R dt H t(x)0.95) 0 no MT resistance
[ [
∫
∫
] ]
which means the effect of mass transfer resistance on the mean hydrogenation rate RHav under x ) 0.95 is
Ind. Eng. Chem. Res., Vol. 39, No. 2, 2000 281
Figure 5. Operation surface.
options, but the computation can be carried out in a similar manner using the same model. Analysis of Kinetic Behavior
Figure 4. Effect of q on reaction trajectories.
limited within 10%. Equation 13 can be reduced to
R h H(x)0.95) )
3 ) 0.9 [θ(x)0.95)]real conditions
(13b)
which is equivalent to
[θ(x)0.95)]real conditions ) 3.33
(13c)
The combination of R, q, and b that satisfies eq 13 can be computed through eq 1. This combination, in fact, is a three-dimensional surface composed of R, q, and b, shown in Figure 5. Much information can be extracted from this surface. If, for example, kinetic conditions and the carbon-carbon double bond loading level are known, i.e., b and q are given, R can be determined from Figure 5. Thus the requirement for mass transfer capacity is known, which is the basic knowledge needed for the design or selection of the hydrogenation reactor. Inversely, if the reactor is given and kinetic parameters are known, which means R and b are given, we could determine the loading level of carbon-carbon double bonds to ensure the effect of the resistance of mass transfer to be limited to less than 10%. Some calculation results for the systems involved are shown in Table 4. For example, if the hydrogenation of PB is operated under the conditions shown in Table 4, the profitable polymer loading concentration should be less than 8.5% to satisfy the objective of eq 13. Certainly, different objectives will result in different preferable condition
A reasonable process should be designed to operate under kinetically controlled conditions. When a reaction is under kinetic control, further improvement of the hydrogenation relies on the optimization of kinetic conditions. Therefore, understanding the kinetic behavior is essential for the design and control of the process. The kinetics for the homogeneous hydrogenation of PB, SB, SBS, and NBR catalyzed by RhCl(PPh3)3 have been investigated experimentally in our laboratory.2-3,8-10 Here the sensitivity of kinetic parameters and selection of kinetic conditions are analyzed to reveal the controlling factors of the reactions and to obtain the principles for the optimization of kinetic behaviors. Sensitivity to Kinetic Parameters and Component Concentration. The effect of the kinetic parameters and the component concentrations on the hydrogenation can be described by the analysis of the sensitivity to the parameters and operation variants. For this purpose, the sensitivity factor Sv of variant v is defined as follows:
dRH RH
)
∑i
Svi
dvi vi
(14)
For the concerned hydrogenation systems, the expressions of the sensitivity factors can be derived as follows:
SK1 ) K′[PPh3][H2](K-1 + [H2])/{K1 + K′[PPh3]/K + K′[PPh3][H2] + K′K1[H2] + mK1K5[CdC]0 + mK′K1K2[H2][CdC]0} (15) SK′ ) K1(1 + mK5[CdC]0)/{K1 + K′[PPh3]/K + K′[PPh3][H2] + K′K1[H2] + mK1K5[CdC]0 + mK′K1K2[H2][CdC]0} (16) SH2 ) {K1 + K′[PPh3]/K + mK1K5[CdC]0}/{K1 + K′[PPh3]/K + K′[PPh3][H2] + K′K1[H2] + mK1K5[CdC]0 + mK′K1K2[H2][CdC]0} (17)
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Table 4. Examples for Selection of Operation Conditions polymer (solvent)
[Rh], mM
T, K
[PPh3], mM
PH2, atm
KLa, s-1
[CdC]0c, mM
1,4PB (o-dicholorobenzene) SB (toluene) SBS (o-dicholorobenzene) NBR87 (butanone) NBR96 (chlorobenzene)
1.99 2.04 1.99 1.958 0.00804
338.2 323.2 338.2 313.2 418.2
7.4 0 7.4 0 4.0
20 20 5 5 24
0.01
1568 (8.5%) 1265 (8.3%) 1447 (10.5%) 1175a (10%) 820a (6.8%)
a
Based on eq 25.
S[PPh3] ) -SK1
(18)
for all systems involved,
SK5 ) -mK1K5[CdC]0/{K1 + K′[PPh3][H2] + K′K1[H2] + mK1K5[CdC]0 + mK′K1K2[H2][CdC]0} (19) for NBR hydrogenation in chrolobenzene,
SK2 ) -mK′K1K2[H2][CdC]0/{K1 + K′[PPh3][H2] + K′K1[H2] + mK1K5[CdC]0 + mK′K1K2[H2][CdC]0} (20) S[CdC]0 ) {K1 + K′[PPh3][H2] + K′K1[H2]}/ {K1 + K′[PPh3][H2] + K′K1[H2] + mK1K5[CdC]0 + mK′K1K2[H2][CdC]0} (21) for NBR hydrogenation in butanone or chrolobenzene and
S[CdC]0 ) 1
(22)
SK ) K′[PPh3]/{KK1 + K′[PPh3] + KK′[PPh3][H2] + KK′K1[H2]} (23) for SB, SBS, and PB. Some explicit results around the typical operation conditions are shown in Table 5. Selection of Kinetic Conditions. According to the analysis to the sensitivity, many operation parameters influence the hydrogenation but the effects are divergent. The selection of some operation conditions is discussed here. First, the effect of [CdC]0 is analyzed, which is generally related to the productivity. It is known from the analysis of sensitivity that the hydrogenation rate of PB, SB, and SBS is proportional to the [CdC]0. Increasing the CdC loading level would always be helpful to heighten the productivity if the transport capacity of the hydrogenation reactors were sufficient. So in these hydrogenation operations it is very important to develop a reactor with highly efficient mass transfer performance. But, S[CdC]0 is less than 1 for NBR hydrogenation, which means the effect of [CdC]0 on the yield decreases as [CdC]0 increases. It is illustrated in Figures 6 and 7 that when [CdC]0 is higher than an appropriate [CdC]0c, there is nearly no effect of increasing [CdC]0 on hydrogenation rate and hydrogenation degree. This is caused by the presence of the CN group in NBR. So increasing [CdC]0 is not always effective to heighten the production capacity of NBR hydrogenation. Moreover, the increases in [CdC]0 would result in increases in viscosity of the reaction fluid, which may worsen mass transfer, mixing, and heat transfer in the reactor.
Second, the effect of increasing hydrogen pressure in the other systems involved is expected to be similar to the effect of [CdC]0 in NBR system because of S[H2] < 1 for all the systems considered. When hydrogen concentration in the liquid phase is higher than a certain [H2]c the increase in hydrogen pressure shows no significant effect on reaction rate (Figure 6) and conversion (Figure 7). But the effect is more important at higher temperatures than that at low temperatures (see Table 5). Finally, the effect of temperature can be roughly estimated from a comparison of the activation energy. The order of the effect for an increase of the operation temperature by 10 °C around the conditions in Table 1 can be obtained as follows under corresponding operation conditions:
NBR87 > 1,4-PB > SBS > T, K 313.2 338.2 338.2 C6H4Cl2 C6H4Cl2 solvent C4H8O SB > NBR96 418.2 324.2 (24) C6H5CH3 C6H5Cl Some interesting information can be obtained from this order, although detailed comparison is not possible for the divergent operation conditions. It seems that the effect of the temperature is largely affected by the level of the operation temperature itself, and it becomes weak as the temperature increases. This is more significant in systems with high steric hindrance. In addition, it can be seen from Table 5 that the influences of [H2] and K′ are higher under higher temperatures than those under low temperatures, but the effect of K1 is inverse. This implies that there may exist an optimal temperature between 40 and 145 °C for the combined contribution of K1 and K′. But in all of the systems the effect of temperature is more effective than [CdC]0 and [H2] in enhancing the hydrogenation reaction. Therefore, the profitable operation conditions should be in the vicinity of [CdC]0c and [H2]c, and under appropriately higher temperature within the tolerable or suitable range for catalyst, polymer, and solvent. [Cd C]0c and [H2]c are mutually affected, which can be decided from Figures 6 and 7 under a given objective. It can be understood when RHav is very near to (RHav)max, the maximal value of RHav, any further attempt to enhance the process would be almost fruitless, in addition to energy-consuming. As an example, [CdC]0c can be determined under the objective
f)
RHav (RHav)max
) 0.9
(25)
The [CdC]0c in NBR hydrogenation is attained at a lower level than that predicted by eq 13. So the [CdC]0 for NBR hydrogenation shown in Table 4 was in fact obtained on the basis of eq 25.
Ind. Eng. Chem. Res., Vol. 39, No. 2, 2000 283 Table 5. Sensitivity of Kinetic Parameters and Operation Conditions polymer (solvent) NBR96 (chlorobenzene) NBR87 (butanone) SBS (o-dicholorobenzene) SB (toluene) 1,4PB (o-dicholorobenzene) a
SK
SK1 ) -SPPh3
SK′
SK 2
SK 5
Sk′
S[CdC]0
SH2
0.61 0.09 (0.034a) 0.07 0.30 (0.18a) 0.11
-0.25 -0.84 (-0.33a)
-0.50
0.34 0 (0.087a) 0.19
0.10 0 (0.60a) 0.75 0 (0.43a) 0.62
1 1 1 1 1
0.25 0.16 1 1 1
0.61 0.09 0.41 0.30 0.30
At [PPh3] ) 4.0 mM.
Figure 6. RHav vs [CdC]0 and [H2] (x ) 0.98, [Rh]T ) 0.080 mM, m ) 0.6245, [PPh3] ) 4.0 mM, T ) 418.2 K, solvent ) C6H5Cl). Solid lines are contours.
Figure 7. x of CdC in NBR vs [CdC]0 and [H2] (t ) 600 s, [Rh]T ) 0.080 mM, m ) 0.6245, [PPh3] ) 4.0 mM, T ) 418.2 K, solvent ) C6H5Cl). Solid lines are contours.
Conclusion and Recommendation A dynamic model for reaction coupled with mass transfer was established for NBR, PB, SB, and SBS hydrogenation under semibatch operation. It is concluded that coupling behaviors of these processes can be depicted by three parameters: the relative capacity of reaction over mass transfer, the ability of catalyst in activating molecular hydrogen, and the CdC loading level. The evolution of the reaction trajectories was investigated, and it was shown that there existed no singular point in the reaction plane and the reaction terminal point was not affected by the starting points. An operation surface was built on which the mass transfer resistance was negligible. The sensitivity of the hydrogenation operation to kinetic parameters and the effect of operation conditions were analyzed. It was
suggested that if the mass transfer performance was adequate, increasing the CdC loading level would be a useful way to heighten the productivity for the hydrogenation of PB, SB, and SBS, but not for the hydrogenation of NBR. According to the analysis carried out above, some further work is suggested in order to heighten the productivity of hydrogenation operation and to reduce the production cost. In the case of PB, SB, and SBS hydrogenation operation, with respect to increasing Cd C loading level in order to gain productivity, development of a highly efficient reactor, which can deal with highly viscous polymer fluid with superior transport capacity, would be very important. Enhancing the transport behavior of a reactor for operation with high polymer concentration is a challenging subject. In fact, the volumetric mass transfer coefficient, used in the computation of this paper and obtained under low polymer content, is quite high. For a commercial scale to reach this mass transfer level is already not easy. It is nearly impossible for a conventional vertical stirred reactor under high polymer concentration to possess the mass transfer behavior superior to the level used here. So some novel reactors with superior transport behavior are required. For NBR hydrogenation, the effect is not very significant with respect to increasing the CdC loading level on the productivity unless the coordination of CN groups to catalytic sites could be eliminated by the design of a more selective catalyst. To reduce or eliminate the use of a large amount of solvent is desirable in all the systems involved, not only for decreasing production cost but also for environmental concerns. Thus the use of aqueous-phase hydrogenation is a possible means to this end, which has been attempted by Mudalige and Rempel12 using water-soluble rhodium catalysts. Such catalyst systems may provide for the integration of the polymerization and the hydrogenation operations into one single process, which would simplify the operation and reduce the production cost. Another way worth considering is a bulk hydrogenation operation assisted by supercritical fluids, which would eliminate the impregnation operation of catalysts by organic solvent required in Gilliom’s method11 and enhance the mass transfer performance. In addition, the rheological behavior of the diene-based polymer solutions under different degrees of hydrogenation and different polymer concentrations needs to be measured. The relationship of the transport behavior in hydrogenation reactors over a wide range of operation conditions and fluid properties also needs to be established for the direct design of an optimal hydrogenation process. Acknowledgment Support from the Natural Science and Engineering Research Council of Canada (NSERC) and from the National Natural Science Foundation of China (NSFC NO. 29676037) to Qinmin Pan is gratefully acknowledged.
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Ind. Eng. Chem. Res., Vol. 39, No. 2, 2000
Nomenclature m-1
a: interfacial area per unit volume of liquid, [A], [B], [C]: catalytic site concentration, shown in Figure 1, mM [A0], [B0], [C0]: catalytic site concentration, shown in Figure 1, mM b: dimensionless parameter, defined in eq 9 [CdC]: concentration of carbon-carbon double bond, mM [CN]: nitrile concentration, mM E: apparent activation energy, kJ/mol f: defined in eq 25 h: dimensionless concentration of hydrogen, defined in eq 5 [H2]: hydrogen concentration, mM K1: reaction equilibrium constant, mM K, K′, K2, K5: reaction equilibrium constants, (mM)-1 k′: kinetic constant, (mM s)-1 kL: mass transfer coefficient in liquid phase, s-1 kLa: volumetric mass transfer coefficient in liquid phase, s-1 KLa: overall volumetric mass transfer coefficient, s-1 m: proportional factor ([CdC]0/[CN]) [PPh3]: triphenylphosphine concentration, mM q: dimensionless parameter, defined in eq 8 R: dimensionless parameter, defined in eq 10 RH: hydrogenation rate, mM/s [Rh]: concentration of Rh-based catalyst, mM S sensitivity factor, define in eq 14 t: time, s T: temperature, K x: conversion θ: dimensionless time, defined in eq 6 Subscripts 0: t)0 av: average c: critical value [CdC]: CdC concentration [CdC]0: CdC concentration at t ) 0 e: equilibrium [H2]: H2 concentration MT: mass transfer max: maximal value re: relative T: total x: conversion
Literature Cited (1) Bhatacharjee, S.; Bhowmick, A. K.; Avasthi, B. N. Preparation of Hydrogenated Nitrile Rubber Using Palladium Acetate Catalyst: Its Characterization and Kinetics. J. Polym. Sci., Part A: Polym. Chem. 1992, 30 (3), 471-484.
(2) Mohammadi, N. A.; Rempel, G. L. Homogeneous Selective Catalytic Hydrogenation of CdC in Acrylonitrile-Butadiene Copolymer. Macromolecules 1987, 20 (10), 2362-2368. (3) Parent, J. S.; McManus, N. T.; Rempel, G. L. RhCl(PPh3)3 and RhH(PPh3)4 Catalyzed Hydrogenation of Acrylonitrile-Butadiene Copolymers. Ind. Eng. Chem. Res. 1996, 35 (12), 4417-4423. (4) Bhatacharjee, S.; Bhowmick, A. K.; Avasthi, B. N. HighPressure Hydrogenation of Nitrile Rubber: Thermodynamics and Kinetics. Ind. Eng. Chem. Res. 1991, 30 (6), 1086-1092. (5) Martin, P.; McManus, N. T.; Rempel, G. L. A Detailed Study of the Hydrogenation of Nitrile-Butadiene Rubber and Other Substrates Catalyzed by Ru(II) Complexes. J. Mol. Catal. A: Chem. 1997, 126 (2-3), 115-131. (6) Parent, J. S.; McManus, N. T.; Rempel, G. L. Hydrogenation of Diene Copolymers. U.S. Patent 5,561,197, 1996. (7) Parent, J. S., McManus, N. T.; Rempel, G. L. OsHCl(CO)(O2)(PCy3)2 Catalyzed Hydrogenation of Acrylonitrile-Butadiene Copolymers. Ind. Eng. Chem. Res. 1998, 37 (11), 4253-4261. (8) Mohammadi, N. A.; Rempel, G. L. Homogeneous Catalytic Hydrogenation of Polybutadiene. J. Mol. Catal. 1989, 50, 259275. (9) Guo, X.; Rempel, G. L. Catalytic Hydrogenation of Diene Polymers: Part I. Kinetic Analysis and Mechanistic Studies on the Hydrogenation of Polybutadiene Polymers in the Presence of RhCl(PPh3)3. J. Mol. Catal. 1990, 63, 279-298. (10) Guo, X.; Parent, J. S.; Rempel, G. L. Catalytic Hydrogenation of Diene Polymers: Part II. Kinetic Analysis and Mechanistic Studies on the Hydrogenation of Styrene-Butadiene Polymers in the Presence of RhCl(PPh3)3. J. Mol. Catal. 1992, 63, 279-298. (11) Gilliom, L. R. Catalytic Hydrogenation of Polymers in the Bulk. Macromolecules 1989, 22, 662-665. (12) Mudalige, D. C.; Rempel, G. L. Aqueous-Phase Hydrogenation of Polybutadiene, Styrene-Butadiene, and Nitrile-Butadiene Polymer Emulsions Catalyzed by Water-Soluble Rhodium Complexes. J. Molecular Catal. A: Chem. 1997, 123, 15-20. (13) McManus, N. T.; Rempel, G. L. Chemical Modification of Polymers: Catalytic Hydrogenation and Related Reactions. J. Macromol. Sci., Rev. Macromol. Chem. Phys. 1995, 35 (2), 239285. (14) Mohammadi, N. A.; Rempel, G. L. Control, Data Acquisition and Analysis of Catalytic Gas-Liquid Mini Slurry Reactors Using a Personal Computer. Comput. Chem. Eng. 1987, 11 (1), 27-35. (15) Danckwerts, P. V. Gas-Liquid Reactions; John Wiley & Sons: New York, 1970; Chapter 5. (16) Parent, J. S. Catalytic Hydrogenation of Butadiene Copolymers. Ph.D. Thesis, Department of Chemical Engineering, University of Waterloo, 1996. (17) Seydel R. Practical Bifurcation and Stability Analysis: from Equilibrium to Chaos; Springer-Verlag: New York, 1994; Chapter 2.
Received for review June 16, 1999 Revised manuscript received November 15, 1999 Accepted November 15, 1999 IE9904347
