Modelling the catalytic hydrochlorination of ethyne

Applied Catalysis, 67 (1990) 159-168 Elsevier Science Publishers B.V., Amsterdam

159

Modelling the catalytic hydrochlorination

of ethyne

Dimitar Iankov Kamenski* and Subcho Dimitrov Dimitrov Higher Institute

of Chemical

Technology,

8010 Bourgas (Bulgaria)

and Lidia Anatol’evna Sil’chenko, Genadi Konstantinovich Shestakov, Konstantin Urevich Odinzov and Oleg Naumovich Temkin Lomonosov

Institute

of Fine Chemical

Technolog),

Moscow

(U.S.S.R.)

(Received 20 July 1989, revised manuscript received 24 May 1990)

Abstract A kinetic model of the homogeneous catalytic hydrochlorination of ethyne is obtained on the basis of kinetic data, reactant solubilities and the phase equilibrium in the HCl-C,H,OH-C,H, system. This model is compared with three simplified rate models by using a sequential discrimination procedure. The first model and one of the simplified models were applied to simulate a continuous stirred tank reactor cascade which includes a tubular plug flow reactor at its end. It is shown that the results produced by the two models do not differ significantly and thus both models may be successfully applied to simulate a plant-scale reactor system. Keywords: hydrochlorination simulation.

of ethyne, kinetics, mathematical modelling, platinum complexes, reactor

INTRODUCTION

Ethyne hydrochlorination is of importance from an industrial point of view for vinyl chloride production. Usually, the reaction is carried out in the gas phase in the 150-200°C range on an active carbon-supported HgClz catalyst. However, the basic disadvantage of the process is the use of an unstable and toxic mercury catalyst. On the other hand the selection of a set of suitable processing conditions for high conversion of reactants leads to a short catalyst life. In industrial vinyl chloride manufacture a large amount of catalyst must be used to ensure a satisfactory life of the catalyst, resulting in an increase in the reactor size. Therefore the mercuric catalyst has no perspective for developing into an industrial process ensuring high productivity and low capital and operating costs, especially when natural gas and coal are to be used as alternative raw materials for vinyl chloride production. Recently, the high catalytic activity of homogeneous platinum complexes in

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160

the hydrochlorination of ethyne has been reported [ 11.In an alcohol medium the reaction proceeds in the O-40’ C range, the catalyst activity reaching up to 300 moles of vinyl chloride per mole catalyst per hour. On the basis of previous kinetic studies of this process [ 231, the experimental results have been fitted to a power rate law (1) by taking account of the reactant solubilities at 40°C and the phase equilibrium in the HCl-C,H,OHC2H2 system. r=GtGzHzGICI

(I)

where r is the reaction rate, k is the rate constant, CPt, CCzH2and CHcl stand for the concentrations of catalyst, ethyne and hydrogen chloride in the solution, respectively. It is well known that kinetic data and models play an important role in designing and analysing a chemical reactor and vinyl chloride synthesis is no exception to this. Unfortunately, the form of the rate Eqn. ( 1) is not convenient for practical use. For such purpose it is more appropriate to correlate the reaction rate with the partial pressures of ethyne and hydrogen chloride. The present study aims to develop a kinetic model which adequately described the homogeneous catalytic hydrochlorination of ethyne. Attention is also directed towards the application of the selected model in determining a CSTR cascade behaviour. EXPERIMENTAL

A laboratory CSTR has been used for obtaining kinetic data under steady state conditions. The gas phase (C,H, + HCl) was continuously passed through the catalyst solution. The high degree of mixing eliminated the mass transfer resistance and the reaction proceeded in the kinetic region. The experiments were carried out under atmospheric pressure and at a temperature of 40°C. It has previously been found [2] that this is the optimum temperature level for the reaction. Two series of experiments were conducted with two different catalyst concentrations: 0.005 and 0.008 mol/l. The partial pressures of ethyne and hydrogen chloride were varied from 5 to 40 kPa and from 10 to 70 kPa, respectively. The product analyses were performed by means of a gas chromatograph, equipped with a thermal conductivity detector and helium as carrier gas. The column was packed with Poropak Q, and it had a diameter of 4 mm and was 3 m long. RESULTS AND DISCUSSION

The experimental data are shown in Table 1.

161 TABLE

1

Experimental

data

Run

x 103

1

PHCl

P C2H2

r

(mol/l)

&Pa)

W’a)

(mol/l.h)

2

3

4

5

c,

1

5.00

58.22

4.20

0.2500

2

5.00 5.00

58.84 58.30

11.34

0.6200

5.00 5.00 8.00

57.95

15.84 23.01

1.0000 1.5000

39.88 6.07

2.3500 0.4900 1.2800 1.5800 1.9400 2.1900

3 4 5 6 7 8

12

8.00 8.00 8.00 8.00 8.00 0.5

13 14 15

9 10 11

57.36 57.37 55.83 54.79 54.83 53.07 54.13

13.25 16.46 19.55 22.90 31.81 40.15

3.0200 0.2500 0.5300

1.00 2.50

57.16 58.38 58.20

41.05 40.76

5.00

57.38

39.88

2.3500

16

8.00

17 18

10.00 1.02 1.02

55.83 54.87

13.25 28.55

1.2800 3.4800

8.39 8.89 12.84 20.22

5.98 6.48 8.83 15.48

0.1000 0.0085

19 20 21 22 23 24

1.02 0.72 0.72 0.72 0.72

20.67 21.48

1.2200

0.0189 0.0440

25.00 25.32 25.82

18.93 15.62 15.34 19.68 18.98

0.0800 0.0700 0.1000 0.1195 0.1100

25 26 27

0.72 0.72 0.72

28

1.02

28.80 32.81

14.59 23.64

0.0670 0.1741

29

1.02

48.80

48.08

30

1.02

66.65

31.36

0.4504 0.3227

31

1.02

74.40

23.79

0.2657

Kinetic analysis The experimental results for the rate of vinyl chloride formation as a function of the partial pressures of reactants have been fitted to four rational function models. This type of rate model was selected since an evaluation of rational functions showed that they are capable of representing a wide range of data forms, fitting data well with a parsimonious number of parameters [4,5]. It has also been found [6] that they tend to provide more accurate extrapola-

162

tion of both the fitted value and the first derivative, which is essential for application of the mathematical models for reactor performance simulation. The four rival rate models which were tested and considered as candidates for discrimination are as follows: Model 1 Model 2 Model3

(1+5.811PHc1)4

r= r=

Model 4

kCp,P~~~(1+1.274PHC1)PC2H2

r=

kCrtPc&‘nc, 1 +K,P,,, kGthd%c~ 1+KP2 1

HCI

kCrtPczuzP& r=l+KIPHcl +K,P&

where r is the reaction rate, k is the rate constant, K, and Kz are constants, Cr, is the concentration of catalyst, and PCzHzand PHcl are the partial pressures of ethyne and hydrogen chloride, respectively. Model 1 is deduced on the basis of rate eqn. (1) and experimental data [ 2,3] obtained at 40°C for the solubility of ethyne and hydrogenchloride in ethanol. These data are correlated using the following equations: CCzHz=

CHCl

=

1

& +K&~PHc, p c~H~[CZH~OHI~ 1+KI~HCI

(f-3)

,“;(‘y [W-WHI, 1

HCl

where [C,H,OH], is the total concentration of ethanol, K,, K2 and K3 are constants (K,=3.24x10-2kPa,K,=3.56x10-4kPaandK,=1.4x10-4kPa). The total concentration of ethanol was expressed as a function of density of solution p and the concentration of hydrogen chloride [C:!H5CH]z=21.74p-0.80CHc,

(3)

Additionally, the density was correlated with the partial pressure of hydrogen chloride. p= 0.947sPgj;

(9)

The values of the coefficients in eqns. (6) - (9) are estimated by least-squares fitting on the basis of the experimental data for reactant solubilities and the phase equilibrium in the HCl-C,H,OH-C,H, system. By taking eqns. (8) and (9) into account eqns. (6) and (7) can be transformed into:

163

(y _ 66.7~~HCl HC1 - 1+ 5.811PH~~ c CzHz

(10)

0.73P$$( 1 + 1.27PHc+ =

1 + 5.811Pnci

(11)

CzHz

Eqns. (1 ), (10) and (11) were combined and the final version of rate model 1, represented by eqn. (2), was obtained. It should be noted that selecting the structures of models 2 and 4 we taken account of the results [ 31 for the order of the homogeneous catalytic hydrochlorination of ethyne obtained on the basis of kinetic data at 40’ C. These results show that the apparent order of reaction with respect to ethyne and catalyst (Pt (II) complexes), respectively, is approximately equal to 1. A complicated apparent order behaviour of reaction with respect to hydrogen chloride, however, is expected to be higher than 1. Kinetic model 3 has been presented in ref. 1. Parameter estimates of these models were obtained on the basis of the kinetic data from Table 1 by using the Nelder-Mead nonlinear programming method [ 71. These estimated are presented in Table 2, along with the model fits. The standard lack-of-fit test showed that all the four models are adequate at the 5% level. Therefore, the best model should be selected by using a sequential discrimination procedure. The discrimination began with ten initial experimental runs, chosen arbitrarily from the data in Table 1. The divergence criterion D proposed by Box and Hill [ 81 was applied to discrimination procedure.

(12)

TABLE 2 Parameter estimates and ‘lack-of-fit’ test Parameter

Model 1

Model 2

Model 3

Model 4

k Kl K2 Sum of squares of residuals Lack of fit ratio

1406.5

5.90 3.12

26.51 18.84

0.0785 3.92

0.0786 3.93

0.0654 3.61

14.76 1.02 12.97 0.0721 3.27

F(29,5; 0.95) =4.5

164

0.4

0.2

0 0

2

4

6

8

IO

Fig. 1. Model probabilities procedure:

a,

model 1; 0,

12

N

14

7~ as a function

of run number

N in the sequential

discrimination

model 2; A, model 3; A, model 4.

where K,i is the prior probability associated with model i after n runs, cr2is the common error variance, rn+ 1 G) I is the estimated value of the dependent variable for the (n + 1 )th experiment under model i and 0” is its variance. The next experimental run was selected with those values of the operational variables which maximized the divergence defined by eqn. (12 ) . Fig. 1 displays the evolution of each model probability as a function of the run number in the sequential model discrimination procedure. It can be seen that after the introduction of about ten more experiments it is possible to discriminate between the models. The probability associated with model 1 is higher than the probabilities of models 2-4. As a result one can assume that model 1 fits the experimental data best. It should be noted that any further improvement of model 1 requires more detailed information on the fundamental chemistry of the reaction under study. Although the data and model fitting procedures are useful in arriving at rate models which best describe the data for kinetics they are not reliable for determining the mechanism of a catalyzed reaction. It is therefore hazardous to push the present kinetic analysis to any conclusion about the detailed mechanism of the reaction. However, the best kinetic model 1 for catalytic hydrogenation of ethyne may be regarded in correspondence with the conventional mechanism of this reaction, including steps for formation and decomposition of intermediates between the catalyst and reactants which contain the fragment tram-Pt (II) - CH=CHCl. Reactor performance simulation A reactor simulation based on the developed mathematical models has been carried out. The favorite model 1 and model 2 were used to simulate the performance of a multiple continuous stirred tank reactor cascade which includes

165

Fig. 2. Cascade of well stirred reactors and a plug flow reactor.

0

I/’ 0

_Ll 5

10

15

2c

25

n

30

Fig. 3. Variation of total system volume V (m3) with inlet pressure PO, (MPa); A, the pressure is constant in the reactor cascade; 0, the volumetric flow rate is constant in the reactor cascade; n = number of well stirred reactors.

one tubular plug flow reactor at its end. Several cases were examined. They correspond to different pressure values (in the range of 0.12 to 0.8 MPa) . This system is shown in Fig. 2. The overall reactor performance may be evaluated by making use of the mathematical reactor models. The equations representing the system of the well mixed reactors are

Vi=GAo(X;-Xi_-l)/ri,

i=l,

2, . ... n

(13)

where Vi is the volume of the ith reactor, GA0are the moles of ethyne per unit time in the feed stream, Xi is the fraction of ethyne which has reacted in the ith reactor (fractional conversion), r’i is the reaction rate in the ith reactor, and n is the number of well-stirred reactors in the cascade. The model for a plug flow reactor is

166 (7 50 F S 40 _

0

30 _ 20 _ 10 _ 0 0

a2

0.4

0.6

0.8

1.0 P, MPa

Fig. 4. Volume V (m3) of a single plug flow reactor as a function of the inlet pressure PO (MPa); A, the pressure is constant in the reactor; 0, the volumetric flow rate is constant in the reactor. 50

“r. 40 9

_

30 _

20 ~

10

0

20

10

0

30

40

50

14,m 3 Fig. 5. Comparison between the total system volume values obtained by Model 1 (V,) and Model 2 ( V,) (0: the pressure is constant in the reactor cascade, A : the volumetric flow rate is constant in the reactor cascade ) .

x,+1

V n+1- -GA,,

s

dx

-r

(14)

n+l

X"

The total system volume was determined for a given overall fractional con-

167

version of ethyne X and a given number of stirred tank reactors with constant volumes of 2 m3. The reactor system performance is estimated in both cases when the pressure or the volumetric flow rate is constant in the reactor cascade. The effect of pressure is considered in assessing the variation of reactor system volume for obtaining 96% fractional conversion of ethyne. The results are shown in Fig. 3. It can be seen that the decrease of pressure is accompanied by a simultaneous increase in the total volume of the reactor cascade. The relatively larger increase of total volume corresponds to lower pressures in the reactor system. On this basis it is possible to specify a suitable number of given sized reactors and an optimum pressure level for obtaining a desirable rate of production. In Fig. 4 we show the reactor volume dependence on inlet pressure POin the case of single plug flow reactor. It is seen that the volume drops rapidly when the pressure increases from 0.12 MPa to 0.3 MPa. Any further increase of pressure results in a negligible variation of the reactor volume. It is interesting to note that the high pressure in the reactor system (about 0.8 MPa) leads to approximately equal total volumes for obtaining 96% overall ethyne conversion in both cases when the pressure or volume flow rate is constant in the reactor system, as can be seen from Figs. 3 and 4. A similar reactor simulation has also been carried out, based on the simplified model 2. This model was found to have probability which is lower than model 1 but higher in comparison with models 3 and 4 (Fig. 1) . The results obtained in this case are not significantly different. As shown in Fig. 5, more significant deviations (about 11% ) between the total system volumes, calculated by models 1 and 2 are obtained only at a low pressure (P z 0.1 MPa). As a result one can assume that the simplified kinetic model 2 can also be successfully applied to simulate the specified plant-scale reactor system.

CONCLUSIONS

A rate model has been obtained to describe the kinetics of catalytic hydrochlorination of ethyne by using information for solubilities of ethyne and hydrogen chloride in ethanol. On the basis of a sequential discrimination procedure it is shown that the model correlates with the experimental data reasonably well. Any further improvement of the model requires more detailed information on the fundamental chemistry of the system under study. A reactor performance simulation has been carried out, based on the developed model and a simplified kinetic model. The results can be used for the arrangement of a multiple continuous stirred tank reactor cascade which includes one tubular plug flow reactor at its end for optimum operation at a given rate of vinyl chloride production.

168

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