Ind. Eng. Chem. Prod. Res. Dev. 1986, 25, 19-22
19
Catalyst Deactivation in Acetylene Hydrochlorination J. B. Agnewt and H. S. Shankar" Department of Chemical Engineering, Monash University, Clayton 3 168, Australia
An experimental study of the deactivation of catalyst (mercuric chloride on activated carbon) used in acetylene hydrochlorination to yield vinyl chloride monomer is described. Deactivation is found to be a first-order process with Arrhenius temperature dependence for deactivation velocity constant.
Introduction Vinvl chloride monomer can be produced bv reacting actylene with hydrogen chloride ove; a catalyst 01mercuric chloride supported on activated carbon. The reaction proceeds at satisfactory rate at temperatures above 100 "C and a t 100-200 kPa pressure. The reaction is highly exothermic (AH, = -110 kJ/mol); industrial production therefore utilizes packed multitubular reactors with external cooling to control temperature rise. Despite such control, hot-spot temperatures generally exceed 140 "C. Above 140 "C catalyst deactivation is known to occur, the rate of deactivation increasing markedly with temperature. Eusuf and Moslem (1967), Webster (1964), and Czarny (1964) have all stated that the large vapor pressure of mercuric chloride above 160 "C leads to significant thermal desorption of mercuric chloride. The catalyst is therefore stated to deactivate; but no quantitative data were reported by any of these investigators. Feldstein (1972) studied the deactivation phenomenon at 180 "C. The results were expressed empirically in terms of a time-varying reaction velocity constant K,: dKr/dt = bp,Kr1.6exp(-4100/RT)
(1)
No experimental details or results were presented. It is clear that there is a paucity of data on catalyst deactivation which would be useful for reactor design and analysis. The aim of the present work was therefore to examine the deactivation phenomenon experimentally and determine an appropriate rate equation. The intrinsic kinetics of the reaction were also studied as a part of this project; details are available elsewhere (Shankar and Agnew, 1980). Equipment A continuous stirred-tank catalytic reactor (CSTCR) was used for the experimental rate determinations. This consisted of a cylindrical 316 stainless steel reactor vessel, 7.4 cm i.d. and 18 cm long, flanged a t both ends. A magnedrive stirrer was fitted to the top lid. Four wire mesh catalyst baskets (11cm X 1.8 cm X 0.7 cm) were fitted to the stirrer shaft of diameter 9.5 mm and propellers were attached above and below the baskets to improve gas mixing. The bottom of the reactor vessel was fitted with a bursting disk made from nickel-plated mild steel (rupture pressure 550 kPa). Separate heating elements were wound on the reactor body, top and bottom flanges, with power inputs regulated individually. The temperature inside the reactor could be measured at a number of points by thermocouples inserted
* Present address: Chemical Engineering Department, I I T Powai, Bombay 400 076, India. +Present address: Department of Chemical Engineering, The University of Adelaide, South Australia, 5001.
in thermowells fitted to the lid. The whole assembly was insulated.
Materials Hydrogen chloride of 99.9% purity was obtained from Matheson Gases Inc., USA. Commercial-grade acetylene was used; this was treated in a purification train containing water, silica gel, and activated carbon to remove impurities before entering the reactor. Vinyl chloride of 99.9% purity was obtained from B. F. Goodrich (Australia) Ltd. for calibration purposes. Tsurumicoal activated carbon was used as a catalyst support; this was obtained from Kogyo Ltd., Japan through I.C.I. Australia Ltd. Three different sizes were used, viz. 4 mm X 4 mm cylinders (TCl), 2 mm X 2 mm cylinders (TC2), and 0.25-0.42-mm granules (TC3). Catalyst Preparation The catalyst was prepared by first dissolving the required quantity of mercuric chloride in distilled water. The required amount of carbon support was then added, and stirring was continued for 24 h. The distribution of mercuric chloride between activated carbon and water is such that almost all the salt adsorbs on the carbon (Glasstone and Lewis, 1966). The carbon-containing mercuric chloride was removed by filtration and dried at 105 "C. The filtrate was then analyzed for mercury by using atomic absorption spectrophotometry to determine, by difference, the quantity of mercuric chloride adsorbed on carbon. Catalyst was prepared separately for each run; the initial mercuric chloride content was kept constant at 100 mg/g of carbon. Analysis Since reported studies indicate that deactivation is due to loss of mercuric chloride from the carbon surface, it was necessary to devise a means of determining the mercuric chloride content of used catalyst. This was achieved by burning samples of catalyst in a bomb calorimeter and then dissolving the residue in dilute acid; the mercury in solution was determined by atomic absorption spectrophotometry. To ascertain the efficiency of mercury recovery, samples of carbon containing accurately known quantities of mercuric chloride were burnt in the same bomb; not less than 99% of initial mercuric chloride was accounted for. The initial mercuric chloride content was generally around 100 mg/g of carbon (shankar, 1976). The mercury content of used catalyst was determined by using the same technique, but it was not possible to identify the chemical environment of the mercury. The ash residue contained small quantities of other inorganic chlorides, so that chloride estimation could not be used. Therefore, wherever mercuric chloride assay was required, the same was estimated by stoichiometry by using data obtained from mercury analysis.
0196-4321/86/1225-0019$01..50/00 1986 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 1, 1986
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Figure 2. Schematic arrangement of reactor, HC1, scrubber, and gas chromatograph.
.^
/ E 0
l 1
\
0.8
ST AND ARD CON DlTlON
L loot---4 REACTOR HEAT-UP TIME, t+-l
Y I w
* I
1-c Figure 1. Schematic representation of reactor heatup. tz.3
t'
0.5
c
0.2
-
1
>
Table I. Experimental Conditions for Deactivation Runs" expt duration, run suuuort temu, "C xu1 h W?, 180 0.12-0.39 17.2 1.02 29 TC 1 210 0.09-0.17 11.3 1.00 20 TC 1 240 8.0 1.02 0.12-0.39 32 TC 1 180 0.12-0.39 42.6 1.00 35 TC2 31 TC2 210 0.12-0.39 22.7 1.00 210 0.12-0.39 22.7 1.00 36 TC2 8 1.19 240 0.12-0.39 17 TC2 240 0.12-0.39 8.3 1.02 30 TC2 0.12-0.39 67.1 0.845 27 TC3 180 210 0.12-0.39 46.8 1.00 25 TC3 240 0.12-0.39 7.9 0.67 28 TC3
t 2 c u d
I
O
0.12-0.39.
The schematic arrangement of the reactor, HC1 scrubber, and gas chromatograph is shown in Figure 2. The rate of reaction r ( t ) at any time t for a well-mixed CSTCR can be given as r ( t ) = nIxV,3(1 - xH1)
(2)
,
.
-
5
3
L
~ 10
TIME, hr
r
-
J
~
r
15
20
Figure 3. Activity-time plot, catalyst TC2, run 36 (symbols as per Table 11). RUN 3 6
T =210°C
Deactivation plots included show behavior in the first 10-15 h of experiment; runs were continued for several more hours.
An F & M Model 700 gas chromatograph fitted with a flame ionization detector was used for gas analysis. Oxydipropionitrile (20% w/w) supported on 80-120 mesh Celite packed in a 120 cm long, 6.3 mm stainless steel tubing was used to achieve the desired separation. Mixtures of acetylene and vinyl chloride required for calibration purposes were prepared by mercury displacement. To avoid corrosion downstream of the reactor, hydrogen chloride was removed from the reactor product stream in an absorber containing potassium hydroxide pellets prior to analysis. Procedure Before a reaction run could be commenced, the reactor had to be heated to the required temperature level. This was done slowly over a period of about 2 h to ensure uniformity of conditions throughout the catalyst. For this reason calculated activities were not based on fresh catalyst but on the rate at the point in time when the desired reaction temperature had been achieved, i.e., at point P in Figure 1. All deactivation experiments were conducted at a stirrer speed of 2500 rpm. Details of experimental conditions are shown in Table I. Wf is the mercuric chloride content of the fresh catalyst used in each run. Deactivation runs were carried out by operating the reactor isothermally at 180, 210, and 240 "C, 1 bar total pressure, and HC1 inlet mole fraction in the HC1-acetylene feed (xH1) in the range
-
1
o'8-
4
-
0.6-
z
3.2
-
Figure 4. Activity-time plot, catalyst TC3, run 25 (symbols as per Table 11).
where n1 (mol/h) is the total inlet molar flow to reactor, xH1 the reactor inlet mole fraction of HC1 in the HC1acetylene feed, and xv3 the vinyl chloride mole fraction in the HC1-free reaction mixture entering the chromatograph. The effect of catalyst deactivation can now be measured by monitoring xv3for chosen values of n, and x H 1 at each temperature. The activity of the catalyst can be defined as a(t) = r(t)/r(td = XVB(~)/XV~(~O) (3) In this work the point t o is chosen as point P in Figure 1; t o is set as zero arbitarily. Results Typical activity-time plots are shown in Figures 3-6. The activity-time relationships can be seen to be independent of the composition of gas in the reactor; the description of symbols for these figures is shown in Table 11.
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Ind. Eng. Chem. Prod. Res. Dev., Val. 25, No. 1, 1986 1.0
I
I
& I
I
T = 210'C
1.0
-I
0.8
I
4
t
I
E X P ( - 0.10 t )
I
1
I
A i
I
I
RUN 28 T = 2 LO°C
1
\
0.6-
>
+ -
2
t a 0.40.2 -
0
4
8 TIME, hr
12
01
I
I
1
1
16
Figure 5. Activity-time plot, catalyst TC3, run 27 (symbols as per Table 11). Table 11. Description of Symbols for Activity-Time Plots svmbol xu1 n,,mol/h 0 0.125 4.26 0 0.146 3.66 A 0.175 3.06 0 0.210 2.46 A 0.280 1.92 0.387 1.38
Figure 6. Activity-time plot, catalyst TC2, run 28 (symbols as per Table 11).
I
I
1
i
SUPPORT- T C 3
-
IL
.c
1
Nonlinear regression analysis showed that the data could be adequately described by exponential decay relationship a ( t ) = a(0) exp(-kdt)
(4)
with activity a(to= 0) set arbitarily at unity for each run. Values of k d for all the runs are shown in column 3 of Table 111. A semilogarithmic plot of k d vs. reciprocal absolute temperature, shown in Figure 7, indicates that the Arrhenius relationship holds for the temperature range examined. The values of k d o and E d are shown in Table IV. In the present work values of k d T were less than 0.005. It is shown by Szepe (1966) that in such a case the process of deactivation is slow compared to time scale for reaction. It is therefore permissible to calculate activities from measurements of reaction rates as a function of time. There was some delay in heating the reactor to the reaction temperature; this heatup time t d varied between 1 and 2 h. Some deactivation occurred during this period. Therefore, in this work activity is defined with respect to reaction rate a t point P in Figure 1and not with respect to fresh catalyst. The activity of the fresh cata1yst.q with respect to activity a ( t o ) has been estimated by using
i
I
0.011 1.9
I
I
1
1
2.0
2.1
2.2
2.3
Figure 7. Arrhenius plot for deactivation velocity constant, kd.
time-temperature data during reactor heatup together with eq 4; these values of af are shown in Table 111. The residual activity a, of used catalyst is shown in column 5 of Table 111. The mercury contents of used catalysts have been determined by methods mentioned earlier. In column 6 of Table I11 these results are shown as the fraction of initial mercury content; initial mercury
Table 111. Results from Deactivation Runs run 29 20 32 35 31 36 17 30 27 25 28 a
NA, not available.
support TC1 TC 1 TC 1 TC2 TC2 TC2 TC2 TC2 TC3 TC3 TC3
T , OC 180 210 240 180 210 210 240 240 180 210 240
I/h 0.06 0.16 0.30 0.053 0.088 0.092 0.205 0.242 0.04 0.102 0.225
kd,
af 1.14 NA" 1.6 1.06 NA" 1.24 1.29 NA" 1.06 1.45 1.32
a,
we/Wf
0.34 NA" 0.08 0.14 0.0 0.10 0.05 0 0.05 0.0 0.04
0.40 NA" 0.02 0.10 0.01 0.06 0.02 0.01 0.05 0.0 0.02
aelaf 0.30 NA" 0.05 0.132 0.0 0.08 0.031 0.0 0.047 0.0 0.03
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 1, 1986
Table IV. Arrhenius Constants for Deactivation catalyst support k d O , l/h Ed, kJ/mol TC 1 2 1 000 41.7 TC2 18 300 48.9 TC3 44 900 52.3 Table V. Enthalpies of Vaporization and Sublimation of Mercury and Mercuric Chloride (Perry. 1963) component AHv,kJ/mol AHs, kJ/mol mercury 57.7 mercuric chloride 58.5 75.2
Acknowledgment H.S.S. wishes to thank Monash University for financial assistance during the course of this study. Glossary a a,
af Ed kd kdO
content was 74 mg/g of carbon in all cases. It can be observed from values of We/W ,as shown in Table 111,that the mercury content of the catalyst decreases markedly with use. It is clear that there is strong correlation between the decrease in mercury content of the catalyst and deactivation. Discussion The internal resistance for the diffusion of mercuric chloride vapor in the pores of carbon was estimated to be small compared to overall resistance for deactivation (Shankar, 1976). This suggests that the observed phenomenon is an intrinsic kinetic process. Feldstein (1972) has suggested a number of possible mechanisms for deactivation without providing supporting evidence. The present work was designed primarily to get a deactivation rate equation for design purposes and was therefore not suitable for elucidating the mechanism of deactivation. The enthalpy of vaporization of mercury and mercuric chloride and the enthalpy of sublimation of mercuric chloride are shown in Table V. The activation energy E d measured in the present study, viz. 47.7-52.3 kJ/mol, is lower than the heat of sublimation of mercuric chloride. It has been shown by Rutner (1964) that the slope of the Arrhenius plot of a nonequilibrium mass-loss process need not be related to the thermodynamic property in question. The deactivation observed in the present work appears to be due to "mass loss" of a mercury species; this is probably a nonequilibrium process and hence the difference between E d and ms. Conclusions (1)Catalyst deactivation can be adequately described by an exponential decay function with velocity constant k, being concentration-independent and showing Arrhenius dependence on temperature. (2) A strong correlation exists between the decrease in mercury content of the catalyst and deactivation.
n1 PA
t td
r T we
Wf X T
ul, AHs
activity defined by eq 2 residual activity activity of fresh catalyst with respect to activity at point P in Figure 1 activation energy for deactivation, kJ/mol deactivation velocity constant, h-' preexponential factor for kd, h-l mol/h acetylene partial pressure, atm time measured with respect to point P in Figure 1, h heatup time, h rate of reaction, mol/h temperature, "C residual mercuric chloride content of catalyst, g mercuric chloride content of fresh catalyst, g mole fraction mean residence time in reactor, h enthalpy of vaporization, kJ/mol enthalpy of sublimation, kJ/mol
Subscripts H hydrogen chloride v vinyl chloride Registry No. Acetylene, 74-86-2;mercuric chloride,7487-94-7; vinyl chloride, 75-01-4.
Literature Cited Czarny, Z. Przem. Cbem. 1964, 43(1), 25. Eusuf, M.; Moslem, M. Sci. Res. (Dacca) 1967, 4 , 111. Feldstein, M. I.Kinet. Catal. (Engl. Trans/.) 1972, 13, 634. Glasstone, S.; Lewis, D. "Elements of Physical Chemistry": Macmillan: London, 1966. Perry, J. H. "Chemical Engineers Handbook", 5th ed.; McGraw-Hill: New York, 1963; pp 3-1 11. Rutner, E.; Goldfuger, P.; Hirth, J. P. "condensation and Evaporation of Solids, Proceedings of the International Symposium on Condensation and Evaporation of Solids", Dayton, OH, Sept 1962; Gordon Breach: New York, 1964. Shankar, H. S. "Kinetics of Acetylene Hydrochlorination", Ph.D. Dissertation, Monash University, Clayton Victoria, Australia, 1976. Shankar, H. S.:Agnew, J. B. Ind. fng. Cbem. Prod. Res. Dev. 1980, 19, 232. Szepe, S. "Deactivation of Catalysts and Some Related Optimisation Problems", Ph.D. Dissertation, Illinois Institute of Technology, Chicago, 1966. Webster, J. W. C. M.Eng. Thesis, University of Melbourne, Australia, 1964.
Received for review March 15, 1985 Accepted September 23, 1985
