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Activity and deactivation of a catalyst for vinyl acetate synthesis in an industrial fluidized reactor
Applied Catalysis, 36 (1988) 67-79
67
Activity and Deactivation of a Catalyst for Vinyl Acetate Synthesis in an Industrial Fluidized Reactor TOSHIO KAWAGUCHI* and THOSHIHISA
WAKASUGI
Kuraray Co., Ltd., Umeda l-12-39, Kitu-ku, Osaka 530 (Japan) (Received 6 May 1986, accepted 20 March 1987)
ABSTRACT The synthesis of vinyl acetate from acetylene and acetic acid has been carried out using two industrial fluidized reactors. Both reactors have an inverted conical shape with a conical angle of 3”20’, and the diameter of the base is 3.28 m. Both have a production capacity of 50 ton/day. An activated carbon supported zinc acetate catalyst of 0.4 mm in average diameter is used. In order to investigate the optimum fluidizing and reaction conditions with the above industrial reactors, a series of operational tests have been carried out over a period of two years. As a result of such operation tests, it has been confirmed that the subject catalyst is free from hysteresis, because, in the operational tests using the catalysts with the same activity level, it shows the same reaction activity under the same reaction conditions, irrespective of the procedure for setting such reaction conditions. Further, it is considered that the rule of additivity of catalyst activity can be applied to this reaction system, because the subject catalyst shows a reaction activity corresponding to its mixing ratio, when using mixtures of catalysts with different activity levels. In these operational tests, we have introduced a method to calculate space time yield (STY) values at a certain past time point, under such conditions that STY changes on standing by a deterioration of catalyst activity. We measured the STY at a certain time point under certain reaction conditions and continued the operation under the same conditions. Several days later, we measured the STY under the same reaction conditions and, then, proceeded with the operational tests under different reaction conditions and measured the STY again. The STY under the latter reaction conditions at the first time point can be estimated by multiplying the ratio of STY actually measured, just before and after changing reaction conditions, and the STY value measured at the first time point.
INTRODUCTION
There have been many studies of reaction conditions for vapour phase reactions using a variety of solid catalysts. However, very few employ large industrial operating equipment. In the case of a fluidized reactor, the reactions are greatly affected by the scale factors for the type of distributor and reaction vessel shape, by fluidizing conditions like L/D, U/Umf, etc. (L: fluidized bed height, D: reactor diameter, U: linear velocity and Umf: minimum fluidizing velocity) as scale effects. Therefore, it is questionable to apply small equip0166-9834/88/$03.50
0 1988 Elsevier Science Publishers B.V.
68
ment test results for a reaction and the optimum conditions obtained from the literature and patents directly to large industrial equipment. We had the opportunity to construct and operate two reactors for the synthesis of vinyl acetate and used these industrial reactors to investigate the optimum reaction conditions and compare the results with those of other studies [l]. Many studies have been made of the synthesis of vinyl acetate from acetylene [ 2-71. However, those reports were obtained using small fixed bed reactors. This study deals with large industrial operating equipment. In analyzing the industrial operation data using large fluidized catalytic reactors for vinylacetate synthesis, it is necessary to solve the problems of changes of space time yield (STY) with time and the deterioration of catalyst activity. The first problem is whether a rise in the STY corresponding to the supply ratio of the more active catalyst is expected or not, when the additionally supplied, more active, catalyst to maintain the production rate is mixed completely with the deteriorated catalyst inside the fluidized catalyst. In this respect, the additive property of the STY should be studied and confirmed. The second problem relates to changes in the catalyst activity with time. In the case of laboratory tests, catalysts of the same activity level, generally new catalysts, are employed so it is possible to compare the reaction results by changing the reaction conditions using the same reactor. In the case of industrial equipment, however, it is only possible to estimate the result with the same activity level, because a large quantity of catalyst is required for one unit. As for this estimation, if there is no hysteresis, it is easy to estimate the STY under different reaction conditions at the same activity level from the change in STY with time. For this, the presence of the hysteresis should be confirmed. In this study, an outline of the manufacturing process and the operating method used in a series of operational tests are described and the additivity results of the STY with additionally supplied catalyst are confirmed. The hysteresis of the STY, when the reaction conditions are changed, is also reported. In addition, the result of studying the method for estimating the STY under different reaction conditions at a certain point is described. EXPERIMENTAL
Manufacturing process and operating method Vinyl acetate is manufactured by the process shown in Fig. 1.The raw material, acetylene, is produced by partial combustion of natural gas with oxygen. It is purified by selective absorption during the process, but, since heavy acetylene compounds are not totally eliminated, for use in the vinyl acetate synthesis, it is further purified by a large continuous moving bed adsorption system using crushed activated carbon [ 81.
69 Zinc
Activated
Crude
acetate
carbon
Acetylene
1
1
1
Catalyst
Gas
Prqaration
Purification 4
Purified
heavy
New Catalyst
Acetylene
acetylene +
W VINYL ACETATE
4 Purged
purified 4
Acetic acid (from polymer process)
SYNTHESIS recovered
gas
Acetic acid W 4
Purification and Recovery
b high boiling waste +by-product Acetaldehyde
VINYL ACETATE
(to polymer process)
Fig.1.Outlineof thevinylacetatemanufacturing process. Because of the fluidized reactor, the inside catalyst is flown off at a rate of about 0.3%/day by wear and crushing. Also, this catalyst has as higher rate of deterioration (approximately OJ%/day) , which necessitates an additional supply of a more active catalyst almost daily. The activated carbon supported zinc acetate catalyst is prepared by the continuous preparation method [ 91 for initial charging and additional supply. Activated carbon of 0.4 mm in average diameter (from crushed palm shell) is used as the supporting material. The amount of zinc acetate supported is 200 kg/catalyst m3; this has been reported as the optimum for the synthesis of vinyl acetate [lo]. In the purification process, the separation and purification of the vinyl acetate produced in the synthetic process and the recovery and purification of unreacted acetic acid are carried out. To feed the synthetic process, the recovered acetic acid is used together with acetic acid recovered from the polyvinyl alcohol manufacturing process through hydrolysis of poly vinyl acetate. Basically, the acetic aced is used in a closed system The vinyl acetate is synthesized in the fluidized reaction vessel in which the catalyst, which has a 24-48 mesh grain size, is fluidized. Using two trains of the same process shown in Fig. 2, vinyl acetate is produced in each at a rate of 50 ton/day. Based on a separate study [ 11, the fluidized reaction vessel was made into an inverted conical shape with a conical angle-of 3 o20’. It is 3.28 m
70
gas scrubbing cyclone
tower
direct condenser
I
ii
-catalyst
i temperature t_...__ ... "....Lar'n .......‘.“‘“.....~..’ .....j :controller
pr e-heater-2
pre-heater-l
I n,.3”“.-3+,?, L
gas blower
Fig. 2. Vinyl acetate synthesis process.
diameter at the base, 3.68 m in diameter at the top, and 7 m high in the fluidizing zone, allowing the maximum amount of catalyst inside to be about 60 m3 in the stationary state of the fluidizing section. The fluidizing gas distribution plates are designed to uniformly distribute gas as well as to support the fluidized catalyst. Based on previous experience, the plates are made by combining eight perforated plates with a rest angle for this catalyst of 40” to keep the catalyst from flowing toward the feed gas pipe even during shutdown. The uppermost perforated plate has 3-mm diameter holes with a 6-mm pitch. The holes of the subsequent perforated plate are positioned so that they are beyond the rest angle, From second plate to eight plate, the hole diameters and pitches are designed to bear the required pressure loss by the plates. The ratio of pressure loss of the distributor to the fluidized bed is set at about 0.3. The reaction temperature is set at 170-2OO”C, taking into account the STY, deterioration rate and impurity of by-product. Since this reaction is exothermic (22 kcal mol-‘), the reaction heat is removed by a liquid phase medium in a jacket. To control the reaction temperature, the cascade system is employed. This means that the output signal from the difference between the internal temperature of the fluidized reactor and the given set temperature is used as the input signal to control the preheater-2 bypass gas flow-rate. By this method the internal temperature can be controlled within -t 05°C at any point in the fluidized section. in
71
Acetylene is circulated by using a Nash type blower with acetic acid as a sealing liquid. After mixing it with acetic acid, evaporated through an evaporator, the feed gas is supplied to the reactor through preheater-l and/or preheater-2. The flown off catalyst with the reaction gas is capturedwith a cyclone. The fine powder catalyst is captured in the gas scrubbing tower by a liquid mainly composed of high boiling compounds. The reaction products such as vinyl acetate, acetaldehyde and unreacted acetic acid are condensed by direct cooling to about 0°C with a cold reaction liquid. The reaction gas is then circulated through the gas blower. In order to maintain the acetylene concentration in the circulating gas, part of this gas is discharged from the reaction system so that the by-product, carbon dioxide, and the inert gas accompanying the catalyst (e.g. nitrogen) are not accumulated. The acetylene, whose quantity corresponds to that of the reacted acetylene, the purged gas and the gas that accompanied the reaction solution, is supplied automatically from the acetylene purification process through a Nash type blower using water as a sealing liquid, so that the inlet pressure of the circulating gas blower is kept constant. The condensed reaction solution is delivered to the subsequent distillation purification process and the fine powder catalyst collecting solution is recovered and purified after the fine powder catalyst is removed by a filter precoated with celite. The additional catalyst, after nitrogen subsitution, is put into the fluidized vessel as a batch from the catalyst supply tank using nitrogen under pressure. Also, as required, the catalyst is taken to the catalyst discharge tank and removed from the system after nitrogen substitution. The powder catalyst captured by the cyclone is also removed form the system in batches. Operation test method and test conditions Catalyst used and estimation of internal catalyst volume The following catalyst is used for the initial supply at start-up and at additional supply point. Zinc acetate supported amount, 200 t 10 g catalyst 1-l; drying loss, under 1.5% by weight; maximum packed bulk density, 0.64-0.72 g cmW3; grain size, 24-48 mesh; average particle diameter, approximately 0.4 mm; minimum fluidizing velocity, approximately 10 cm s-l. The inside volume of the catalyst is based on the stationary state estimated from the balance of the catalyst supplied, discharged and captured by the cyclone and the scrubber and from the change in pressure loss of the fluidized bed. The actual measurement of the internal volume at shutdown after a series of operations is checked. Concentration of acetic acid and acetylene supplied The acetic acid supplied is mainly composed of recovered and purified acetic acid. It has a purity of 99% and contains the following impurities: acetic an-
72
hydride, 0.5-0.7% by weight; ethylidene diacetate, approx. 0.1%; croton aldehyde, approx. 0.1%. The acetylene supplied is almost saturated with moisture at atmospheric temperature since it is delivered by the Nash blower using water as a sealing liquid. If water is not taken into account, the acetylene has a purity of over 99.5% and contains the following impurities: methyl acetylene and/or propadiene, 0.05-0.3% by volume; nitrogen and carbon dioxide, O.l-0.3%. The circulating gas in the reaction system is regulated by discharging gas in order to maintain an acetylene concentration of 92%. Reaction conditions
The reaction temperature was 170-200°C the acetylene molar ratio to acetic acid (MR) was 2.0-3.5 (at the reactor inlet) and the space velocity based on the catalyst volume as the stationary state (SV) was loo-160 h-l. Method
of calculating STY
The integrated value of the acetylene supplied during 1 h is corected for moisture. This value is then made the daily integrated value, and the amount of discharged gas and reaction solution accompanying gas is subtracted from it. The resulting balance is the acetylene consumption. From this acetylene consumption and the acetylene selectivity to vinyl acetate, the amount of vinyl acetate production per day is estimated. The acetylene selectivity is 95-96.5% in accordance with the data already accumulated. In the STY (y) calculation it has a constant value of 96%. The selectivity to other impurities is as follows (based on mole) : acetaldehyde, approx. 3.0% (depending on moisture in the acetylene) ; acetone, 0.2%; croton aldehyde, 0.2%; acetic anhydride, 0.4%; ethylidene diacetate, 0.4%. The estimated amount of vinyl acetate production is checked by the quantity of production in the subsequent distillation purifying process. Although many errors are found over a short period of time, the total estimated amount for one month agrees very well with the actual data, Using this etimated amount of daily vinyl acetate production and the volume of catalyst based on that in the stationary state, the STY (y) (ton-day-lacatalyst m-“) was calculated. RESULTS
Property
of hysteresis
The reaction activity of a catalyst is not always the same, even under the same reaction conditions. It may be influenced by the reaction conditions of previous operations. We call this phenomenon hysteresis. A simple test was carried out on the activated carbon supported zinc acetate catalyst. While the
73
g +J
u” i, d s * s s u” fo % .E z
1.2
0
174
2.5
100
A
180
2.5
100
0
174
2.5
100
1.0
0.8
0
1
5
2 Time
3
4
5
6
7
8
( day 1
Fig. 3. Hysteresis property test.
test operation was running under fixed reaction conditions, the test (with different reaction conditions) interrupts the running test operation for a short period. The tests under different reaction conditions were carried out twice, as shown in Fig. 3, for a day in which a drop in the catalyst activity by deterioration can be neglected. The STY observed was seen to be unstable during testing because of its short time. The plotted value of Fig. 3 is obtained from the acetylene consumption (per every 8 h) which is converted to the STY. This suggests that the observed STY might be false. However, the STY of the test operation run under identical reaction conditions was stable and almost the same before and after the test using different reaction conditions. Therefore, it is our opinion that this catalyst does not exhibit hysteresis. Additiveproperty
of STY
The additive property of the STY was studied by the change in STY indicated by the catalyst in the fluidized reactor when a more active catalyst, (i.e., a new catalyst), was additially supplied. From Fig. 4 it is confirmed that a rise in STY is observed, related to the ratio of addition of active catalyst. This is called the additivity of the STY. This plot was obtained from the acetylene consumption every 8 h by the same method shown in Fig. 3 above. Taking into account the influence of fluidized conditions of L/D and U/Urn/, the amount of the additional supply catalyst should not be too high. With the accuracy of STY estimation, the tests were conducted with an additional supply ratio of 5% and lo%, respectively. For the additional supply, the new catalyst was more active and its STY was estimated to be 1.30 under the reaction conditions used. The amount of the feed gas of acetylene and acetic acid was increased when the new catalyst was
..---.._..-I.
2.35 m3 additional supplied
(1)
1
i
2 Time
4
(
day
6
)
Fig. 4. Additivity of STY (semi-log scale) _
added, and the tests were carried out with fixed SV, MR and reaction temperature. As shown in Fig. 4 and by the following calculation, the estimated and observed values are in good agreement. (0.88 ton-day-’ X
.rn-’ x45 m3 + 1.30 ton-day-’
2.35 m3) /47 m3 = 0.91 ton-day-’
(0.90x46.5*+1.30x5.2)/51.5=0.94
em-’
-mm’ (observed value 0.91)
(observedvalue
0.94)
(1) (2)
Over a practical working range, the additivity of the STY values correlate well. How to estimate the STY at the same time and same activity level Unlike testing and studying on a laboratory scale, operational testing with the large fluidized reactor is supposed to be done only once at an initial startup at the same activity level with a new catalyst. It is impossible to test various factors at the same activity level. In this series of tests, some studies have been carried out on how to estimate the STY at the same time and same activity level. ‘The flying off rate of the fine catalyst from the fluidized reactor is taken into account with O.S%/day.
75
I
4
SV 120
h-1
MR
2.5
inside catalyst volume 47 m3
46 m3
45 m3
I 20
a 10 Time
(
day
)
Fig.5. Changeof STY withtime (semi-logscale).
With the reaction activity represented by the STY, its change with time is reported as a deterioration rate in the following equation [ 41. -d(STY)/dB=K;(STY)
(3)
K,, represents a deterioration rate constant with dimension of day-‘. The results of the tests are shown in a In STY-time plot in Figs. 5 and 6. They show a generally good linearity although some variation is present. Fig. 6 shows the result of additionally supplying a more active catalyst. Since the result is the same as shown in Fig. 5, where more active catalyst is not supplied, it is considered that the following equation is established at a certain replacement rate similar to eqn. ( 3) [ 111. -d(STY)/dO=K;(STY)
(4)
K, also represents the declining rate constant for a certain replacement rate of the more active catalyst. Its dimension is also day-l similar to eqn. (3 ) . Fig. 7 shows the modeling of Figs. 5 and 6. The reaction conditions of run A and run C are identical although their STY values are different. The re-test of run A is run C, and the test of run B under different reaction conditions is carried out in between. As mentioned earlier, no hysteresis has been detected in the system following a change in reaction conditions. This was confirmed in Figs. 5 and 6. And so, values of In yoA, ln yen, In yen and In yet form a parallelogram, see Fig. 7.
76
SV 150 h-1
RR 2.5
new catalyst additional 0.4 m3.day-l SGPPlY
1.4
inside catalyst volume
0.8
Time
(
45 m3
day )
Fig. 6. Change of STY with time in case of additional supply of new catalyst (semi-log scale).
condition:
reaction c
A
Time
C
I3 0 ( day
0
)
Fig. 7. Model of STY change for estimation at the same time.
77
TABLE 1 Results of a series of operational tests yI and y2 represents STY observed at starting and ending of each test. yX represents STY estimated at the beginning of each respective run except run No. 6 and 7. Run Catalyst Lfl? No. volume (m”)
React. temp. (“C)
SV (l/h)
MR
Replacement Test STY (ton/day*m3) rate by new period catalyst (day) Starting Ending Estimation (l/day) (Yl) (Y2) (YX)
38.4 39.7 38.6 36.1 33.8
1.29 1.34 1.29 1.23 1.19
170 172 175 177.5 180
142 137 142 152 162
2.5 2.5 2.5 2.5 2.5
-
6 15 27 18 8
1.113 1.185 1.338 1.358 1.425
1.104 1.149 1.251 1.281 1.370
1.11 1.19 1.39 1.50 1.67
55.7 55.9 56.2
1.83 1.83 1.83
174 174 174
100 100 100
2.5 3.0 3.5
0.0027 0.0027 0.0027
12 12 5
0.980 1.002 0.993
0.948 0.920 0.920
0.98 1.03 1.12
57.0 57.0 57.0 56.5 55.7 55.0 54.0
1.87 1.87 1.87 1.85 1.83 1.81 1.75
177.5 180 182.5 185 187.5 190 192.5
100 100 100 102 104 105 108
2.5 2.5 2.5 2.5 2.5 2.5 2.5
-
7 8 11 11 7 5 8
0.886 0.950 0.983 1.011 0.983 1.010 1.032
0.861 0889 0.912 0.914 0.927 0.956 0.934
0.89 0.98 1.09 1.21 1.30 1.40 1.51
64.0 57.0 52.0 49.0
2.09 1.84 1.73 1.63
174 174 174 174
100 100 100 100
2.5 2.5 2.5 2.5
-
8 11 12 6
0.950 0.967 0.942 0.903
0.926 0.935 0.900 0.892
0.95 0.99 1.00 1.01
50.0 49.0 47.5 45.5
1.66 1.63 1.59 1.52
174 174 174 174
100 120 140 100
2.5 2.5 2.5 2.5
-
9 10 13 9
1.028 1.037 1.043 0.985
0.986 1.014 1.024 0.958
1.03 1.08 1.11 1.06
61.0 55.0
2.00 1.81
174 174
100 100
2.5 2.5
0.0137 0.0103
14 14
0.891 0.934
0.918 0.933
0.92 (y,) 0.93 (Yl)
47.0 43.5
1.57 1.45
174 174
120 120
2.5 2.5
-
6 11
1.229 1.125
1.206 1.105
1.21 1.13
-
Therefore, it is assumed that the STY of 0~0 the observed yet 0 days later as follows: In %B
-Y&2
YOA =
fYOBhB)
= In YOB XYI3C
In YOA =YOC
(yoC) can be estimated
(Yz) (Yl)
from
(5)
78
The estimation method at the same time and same activity level obtained from the STY ratio around the change of reaction conditions described by eqn. ( 5) is confirmed and considered applicable to other results of this series of operational tests. These results are shown in Table 1. CONCLUSION
In order to investigate the optimum fluidizing conditions and reaction conditions of the fluidized reactor for the synthesis of vinyl acetate from acetylene, the method of analysis of the operational data obtained using a large industrial fluidized reactor was studied. For utilization of the operational data, it is considered necessary to establish the estimation method of the STY indicated by the mixed catalyst when catalysts of different catalytic activity are additionally supplied and also the estimation method of the STY at the same time and same activity level. With regard to these points, from In (STY) -time plots, utilizing the fact that the deterioration rate of the catalyst activity is proportional to the STY, a rise in STY corresponding to an additional supply ratio of more active catalysts was observed. The additivity of the STY was confirmed. Also, using a ratio of the STY around the change of reaction conditions, the estimation formula for the STY at the same time and same activity level was determined. At the same time and same activity level, the respective STY values corresponding to the respective reaction conditions were estimated and obtained. This provides useful basic data for studying optimum reaction and fluidizing conditions. Reports describing the results of optimum reaction and fluidizing conditions using these data will be published shortly [ 12,131.
ACKNOWLEDGEMENTS
The authors are grateful to the staff of the manufacturing section of the Nakajyo plant of Kuraray Co., Ltd. for operating the equipment to provide them with the operational data for this report, as well as the management of the company for permitting us to use this data. Also, we would like to thank Professor Yichi Murakami of Nagoya University for giving advice and instruction for the preparation of this report. REFERENCES
1 2 3
T. Kawaguchi, H. Kimura and T. Wakasugi, Sekiyu Gakkaishi, 29 (1986 ) 168. A. Mitsutani, Shyokubai (CatalystO, 4 (1962) 127. A. Mitsutani and T. Kominami, J. Chem. Sot. Jpn. Pure Chem. Sec., 80 (1959) 886.
79
A. Mitsutani and T. Kominami, J. Chem. Sot. Jpn. Pure Chem. Sec., 80 (1959) 893. A. Mitsutani and T. Kominami, J. Chem. Sot. Jpn. Pure Chem. Sec., 80 (1959) 890. A. Mitsutani and T. Kominami, J. Chem. Sot. Jpn. Pure Chem. Sec., 80 (1959) 895. S. Yano and S. Matsumoto, J. Chem. Sot. Jpn. Ind. Chem. Sec., 54 (1952) 498. T. Kawaguchi, G. Hira, T. Wakasugi and T. Tatsumi, Kagaku Kogaku Ronbunshu, 11 (1985) 708. 9 T. Kawaguchi and T. Wakasugi, in preparation. 10 T. Kawaguchi, J. Nakagawa and T. Wakasugi, Appl. Catal., 32 (1987) 23. 11 T. Kawaguchi and T. Wakasugi, Kagaku Kogaku Ronbunshu, 12 (1986) 499. 12 T. Kawaguchi, T. Wakasugi and T. Wakasugi and N. Hatano, KagakuKogaku Ronbunshu, 12 (1986) 133. 13 T. Kawaguchi and T. Wakasugi, Kagaku Kogaku Ronbunshu, 12 (1986) 627.
67
Activity and Deactivation of a Catalyst for Vinyl Acetate Synthesis in an Industrial Fluidized Reactor TOSHIO KAWAGUCHI* and THOSHIHISA
WAKASUGI
Kuraray Co., Ltd., Umeda l-12-39, Kitu-ku, Osaka 530 (Japan) (Received 6 May 1986, accepted 20 March 1987)
ABSTRACT The synthesis of vinyl acetate from acetylene and acetic acid has been carried out using two industrial fluidized reactors. Both reactors have an inverted conical shape with a conical angle of 3”20’, and the diameter of the base is 3.28 m. Both have a production capacity of 50 ton/day. An activated carbon supported zinc acetate catalyst of 0.4 mm in average diameter is used. In order to investigate the optimum fluidizing and reaction conditions with the above industrial reactors, a series of operational tests have been carried out over a period of two years. As a result of such operation tests, it has been confirmed that the subject catalyst is free from hysteresis, because, in the operational tests using the catalysts with the same activity level, it shows the same reaction activity under the same reaction conditions, irrespective of the procedure for setting such reaction conditions. Further, it is considered that the rule of additivity of catalyst activity can be applied to this reaction system, because the subject catalyst shows a reaction activity corresponding to its mixing ratio, when using mixtures of catalysts with different activity levels. In these operational tests, we have introduced a method to calculate space time yield (STY) values at a certain past time point, under such conditions that STY changes on standing by a deterioration of catalyst activity. We measured the STY at a certain time point under certain reaction conditions and continued the operation under the same conditions. Several days later, we measured the STY under the same reaction conditions and, then, proceeded with the operational tests under different reaction conditions and measured the STY again. The STY under the latter reaction conditions at the first time point can be estimated by multiplying the ratio of STY actually measured, just before and after changing reaction conditions, and the STY value measured at the first time point.
INTRODUCTION
There have been many studies of reaction conditions for vapour phase reactions using a variety of solid catalysts. However, very few employ large industrial operating equipment. In the case of a fluidized reactor, the reactions are greatly affected by the scale factors for the type of distributor and reaction vessel shape, by fluidizing conditions like L/D, U/Umf, etc. (L: fluidized bed height, D: reactor diameter, U: linear velocity and Umf: minimum fluidizing velocity) as scale effects. Therefore, it is questionable to apply small equip0166-9834/88/$03.50
0 1988 Elsevier Science Publishers B.V.
68
ment test results for a reaction and the optimum conditions obtained from the literature and patents directly to large industrial equipment. We had the opportunity to construct and operate two reactors for the synthesis of vinyl acetate and used these industrial reactors to investigate the optimum reaction conditions and compare the results with those of other studies [l]. Many studies have been made of the synthesis of vinyl acetate from acetylene [ 2-71. However, those reports were obtained using small fixed bed reactors. This study deals with large industrial operating equipment. In analyzing the industrial operation data using large fluidized catalytic reactors for vinylacetate synthesis, it is necessary to solve the problems of changes of space time yield (STY) with time and the deterioration of catalyst activity. The first problem is whether a rise in the STY corresponding to the supply ratio of the more active catalyst is expected or not, when the additionally supplied, more active, catalyst to maintain the production rate is mixed completely with the deteriorated catalyst inside the fluidized catalyst. In this respect, the additive property of the STY should be studied and confirmed. The second problem relates to changes in the catalyst activity with time. In the case of laboratory tests, catalysts of the same activity level, generally new catalysts, are employed so it is possible to compare the reaction results by changing the reaction conditions using the same reactor. In the case of industrial equipment, however, it is only possible to estimate the result with the same activity level, because a large quantity of catalyst is required for one unit. As for this estimation, if there is no hysteresis, it is easy to estimate the STY under different reaction conditions at the same activity level from the change in STY with time. For this, the presence of the hysteresis should be confirmed. In this study, an outline of the manufacturing process and the operating method used in a series of operational tests are described and the additivity results of the STY with additionally supplied catalyst are confirmed. The hysteresis of the STY, when the reaction conditions are changed, is also reported. In addition, the result of studying the method for estimating the STY under different reaction conditions at a certain point is described. EXPERIMENTAL
Manufacturing process and operating method Vinyl acetate is manufactured by the process shown in Fig. 1.The raw material, acetylene, is produced by partial combustion of natural gas with oxygen. It is purified by selective absorption during the process, but, since heavy acetylene compounds are not totally eliminated, for use in the vinyl acetate synthesis, it is further purified by a large continuous moving bed adsorption system using crushed activated carbon [ 81.
69 Zinc
Activated
Crude
acetate
carbon
Acetylene
1
1
1
Catalyst
Gas
Prqaration
Purification 4
Purified
heavy
New Catalyst
Acetylene
acetylene +
W VINYL ACETATE
4 Purged
purified 4
Acetic acid (from polymer process)
SYNTHESIS recovered
gas
Acetic acid W 4
Purification and Recovery
b high boiling waste +by-product Acetaldehyde
VINYL ACETATE
(to polymer process)
Fig.1.Outlineof thevinylacetatemanufacturing process. Because of the fluidized reactor, the inside catalyst is flown off at a rate of about 0.3%/day by wear and crushing. Also, this catalyst has as higher rate of deterioration (approximately OJ%/day) , which necessitates an additional supply of a more active catalyst almost daily. The activated carbon supported zinc acetate catalyst is prepared by the continuous preparation method [ 91 for initial charging and additional supply. Activated carbon of 0.4 mm in average diameter (from crushed palm shell) is used as the supporting material. The amount of zinc acetate supported is 200 kg/catalyst m3; this has been reported as the optimum for the synthesis of vinyl acetate [lo]. In the purification process, the separation and purification of the vinyl acetate produced in the synthetic process and the recovery and purification of unreacted acetic acid are carried out. To feed the synthetic process, the recovered acetic acid is used together with acetic acid recovered from the polyvinyl alcohol manufacturing process through hydrolysis of poly vinyl acetate. Basically, the acetic aced is used in a closed system The vinyl acetate is synthesized in the fluidized reaction vessel in which the catalyst, which has a 24-48 mesh grain size, is fluidized. Using two trains of the same process shown in Fig. 2, vinyl acetate is produced in each at a rate of 50 ton/day. Based on a separate study [ 11, the fluidized reaction vessel was made into an inverted conical shape with a conical angle-of 3 o20’. It is 3.28 m
70
gas scrubbing cyclone
tower
direct condenser
I
ii
-catalyst
i temperature t_...__ ... "....Lar'n .......‘.“‘“.....~..’ .....j :controller
pr e-heater-2
pre-heater-l
I n,.3”“.-3+,?, L
gas blower
Fig. 2. Vinyl acetate synthesis process.
diameter at the base, 3.68 m in diameter at the top, and 7 m high in the fluidizing zone, allowing the maximum amount of catalyst inside to be about 60 m3 in the stationary state of the fluidizing section. The fluidizing gas distribution plates are designed to uniformly distribute gas as well as to support the fluidized catalyst. Based on previous experience, the plates are made by combining eight perforated plates with a rest angle for this catalyst of 40” to keep the catalyst from flowing toward the feed gas pipe even during shutdown. The uppermost perforated plate has 3-mm diameter holes with a 6-mm pitch. The holes of the subsequent perforated plate are positioned so that they are beyond the rest angle, From second plate to eight plate, the hole diameters and pitches are designed to bear the required pressure loss by the plates. The ratio of pressure loss of the distributor to the fluidized bed is set at about 0.3. The reaction temperature is set at 170-2OO”C, taking into account the STY, deterioration rate and impurity of by-product. Since this reaction is exothermic (22 kcal mol-‘), the reaction heat is removed by a liquid phase medium in a jacket. To control the reaction temperature, the cascade system is employed. This means that the output signal from the difference between the internal temperature of the fluidized reactor and the given set temperature is used as the input signal to control the preheater-2 bypass gas flow-rate. By this method the internal temperature can be controlled within -t 05°C at any point in the fluidized section. in
71
Acetylene is circulated by using a Nash type blower with acetic acid as a sealing liquid. After mixing it with acetic acid, evaporated through an evaporator, the feed gas is supplied to the reactor through preheater-l and/or preheater-2. The flown off catalyst with the reaction gas is capturedwith a cyclone. The fine powder catalyst is captured in the gas scrubbing tower by a liquid mainly composed of high boiling compounds. The reaction products such as vinyl acetate, acetaldehyde and unreacted acetic acid are condensed by direct cooling to about 0°C with a cold reaction liquid. The reaction gas is then circulated through the gas blower. In order to maintain the acetylene concentration in the circulating gas, part of this gas is discharged from the reaction system so that the by-product, carbon dioxide, and the inert gas accompanying the catalyst (e.g. nitrogen) are not accumulated. The acetylene, whose quantity corresponds to that of the reacted acetylene, the purged gas and the gas that accompanied the reaction solution, is supplied automatically from the acetylene purification process through a Nash type blower using water as a sealing liquid, so that the inlet pressure of the circulating gas blower is kept constant. The condensed reaction solution is delivered to the subsequent distillation purification process and the fine powder catalyst collecting solution is recovered and purified after the fine powder catalyst is removed by a filter precoated with celite. The additional catalyst, after nitrogen subsitution, is put into the fluidized vessel as a batch from the catalyst supply tank using nitrogen under pressure. Also, as required, the catalyst is taken to the catalyst discharge tank and removed from the system after nitrogen substitution. The powder catalyst captured by the cyclone is also removed form the system in batches. Operation test method and test conditions Catalyst used and estimation of internal catalyst volume The following catalyst is used for the initial supply at start-up and at additional supply point. Zinc acetate supported amount, 200 t 10 g catalyst 1-l; drying loss, under 1.5% by weight; maximum packed bulk density, 0.64-0.72 g cmW3; grain size, 24-48 mesh; average particle diameter, approximately 0.4 mm; minimum fluidizing velocity, approximately 10 cm s-l. The inside volume of the catalyst is based on the stationary state estimated from the balance of the catalyst supplied, discharged and captured by the cyclone and the scrubber and from the change in pressure loss of the fluidized bed. The actual measurement of the internal volume at shutdown after a series of operations is checked. Concentration of acetic acid and acetylene supplied The acetic acid supplied is mainly composed of recovered and purified acetic acid. It has a purity of 99% and contains the following impurities: acetic an-
72
hydride, 0.5-0.7% by weight; ethylidene diacetate, approx. 0.1%; croton aldehyde, approx. 0.1%. The acetylene supplied is almost saturated with moisture at atmospheric temperature since it is delivered by the Nash blower using water as a sealing liquid. If water is not taken into account, the acetylene has a purity of over 99.5% and contains the following impurities: methyl acetylene and/or propadiene, 0.05-0.3% by volume; nitrogen and carbon dioxide, O.l-0.3%. The circulating gas in the reaction system is regulated by discharging gas in order to maintain an acetylene concentration of 92%. Reaction conditions
The reaction temperature was 170-200°C the acetylene molar ratio to acetic acid (MR) was 2.0-3.5 (at the reactor inlet) and the space velocity based on the catalyst volume as the stationary state (SV) was loo-160 h-l. Method
of calculating STY
The integrated value of the acetylene supplied during 1 h is corected for moisture. This value is then made the daily integrated value, and the amount of discharged gas and reaction solution accompanying gas is subtracted from it. The resulting balance is the acetylene consumption. From this acetylene consumption and the acetylene selectivity to vinyl acetate, the amount of vinyl acetate production per day is estimated. The acetylene selectivity is 95-96.5% in accordance with the data already accumulated. In the STY (y) calculation it has a constant value of 96%. The selectivity to other impurities is as follows (based on mole) : acetaldehyde, approx. 3.0% (depending on moisture in the acetylene) ; acetone, 0.2%; croton aldehyde, 0.2%; acetic anhydride, 0.4%; ethylidene diacetate, 0.4%. The estimated amount of vinyl acetate production is checked by the quantity of production in the subsequent distillation purifying process. Although many errors are found over a short period of time, the total estimated amount for one month agrees very well with the actual data, Using this etimated amount of daily vinyl acetate production and the volume of catalyst based on that in the stationary state, the STY (y) (ton-day-lacatalyst m-“) was calculated. RESULTS
Property
of hysteresis
The reaction activity of a catalyst is not always the same, even under the same reaction conditions. It may be influenced by the reaction conditions of previous operations. We call this phenomenon hysteresis. A simple test was carried out on the activated carbon supported zinc acetate catalyst. While the
73
g +J
u” i, d s * s s u” fo % .E z
1.2
0
174
2.5
100
A
180
2.5
100
0
174
2.5
100
1.0
0.8
0
1
5
2 Time
3
4
5
6
7
8
( day 1
Fig. 3. Hysteresis property test.
test operation was running under fixed reaction conditions, the test (with different reaction conditions) interrupts the running test operation for a short period. The tests under different reaction conditions were carried out twice, as shown in Fig. 3, for a day in which a drop in the catalyst activity by deterioration can be neglected. The STY observed was seen to be unstable during testing because of its short time. The plotted value of Fig. 3 is obtained from the acetylene consumption (per every 8 h) which is converted to the STY. This suggests that the observed STY might be false. However, the STY of the test operation run under identical reaction conditions was stable and almost the same before and after the test using different reaction conditions. Therefore, it is our opinion that this catalyst does not exhibit hysteresis. Additiveproperty
of STY
The additive property of the STY was studied by the change in STY indicated by the catalyst in the fluidized reactor when a more active catalyst, (i.e., a new catalyst), was additially supplied. From Fig. 4 it is confirmed that a rise in STY is observed, related to the ratio of addition of active catalyst. This is called the additivity of the STY. This plot was obtained from the acetylene consumption every 8 h by the same method shown in Fig. 3 above. Taking into account the influence of fluidized conditions of L/D and U/Urn/, the amount of the additional supply catalyst should not be too high. With the accuracy of STY estimation, the tests were conducted with an additional supply ratio of 5% and lo%, respectively. For the additional supply, the new catalyst was more active and its STY was estimated to be 1.30 under the reaction conditions used. The amount of the feed gas of acetylene and acetic acid was increased when the new catalyst was
..---.._..-I.
2.35 m3 additional supplied
(1)
1
i
2 Time
4
(
day
6
)
Fig. 4. Additivity of STY (semi-log scale) _
added, and the tests were carried out with fixed SV, MR and reaction temperature. As shown in Fig. 4 and by the following calculation, the estimated and observed values are in good agreement. (0.88 ton-day-’ X
.rn-’ x45 m3 + 1.30 ton-day-’
2.35 m3) /47 m3 = 0.91 ton-day-’
(0.90x46.5*+1.30x5.2)/51.5=0.94
em-’
-mm’ (observed value 0.91)
(observedvalue
0.94)
(1) (2)
Over a practical working range, the additivity of the STY values correlate well. How to estimate the STY at the same time and same activity level Unlike testing and studying on a laboratory scale, operational testing with the large fluidized reactor is supposed to be done only once at an initial startup at the same activity level with a new catalyst. It is impossible to test various factors at the same activity level. In this series of tests, some studies have been carried out on how to estimate the STY at the same time and same activity level. ‘The flying off rate of the fine catalyst from the fluidized reactor is taken into account with O.S%/day.
75
I
4
SV 120
h-1
MR
2.5
inside catalyst volume 47 m3
46 m3
45 m3
I 20
a 10 Time
(
day
)
Fig.5. Changeof STY withtime (semi-logscale).
With the reaction activity represented by the STY, its change with time is reported as a deterioration rate in the following equation [ 41. -d(STY)/dB=K;(STY)
(3)
K,, represents a deterioration rate constant with dimension of day-‘. The results of the tests are shown in a In STY-time plot in Figs. 5 and 6. They show a generally good linearity although some variation is present. Fig. 6 shows the result of additionally supplying a more active catalyst. Since the result is the same as shown in Fig. 5, where more active catalyst is not supplied, it is considered that the following equation is established at a certain replacement rate similar to eqn. ( 3) [ 111. -d(STY)/dO=K;(STY)
(4)
K, also represents the declining rate constant for a certain replacement rate of the more active catalyst. Its dimension is also day-l similar to eqn. (3 ) . Fig. 7 shows the modeling of Figs. 5 and 6. The reaction conditions of run A and run C are identical although their STY values are different. The re-test of run A is run C, and the test of run B under different reaction conditions is carried out in between. As mentioned earlier, no hysteresis has been detected in the system following a change in reaction conditions. This was confirmed in Figs. 5 and 6. And so, values of In yoA, ln yen, In yen and In yet form a parallelogram, see Fig. 7.
76
SV 150 h-1
RR 2.5
new catalyst additional 0.4 m3.day-l SGPPlY
1.4
inside catalyst volume
0.8
Time
(
45 m3
day )
Fig. 6. Change of STY with time in case of additional supply of new catalyst (semi-log scale).
condition:
reaction c
A
Time
C
I3 0 ( day
0
)
Fig. 7. Model of STY change for estimation at the same time.
77
TABLE 1 Results of a series of operational tests yI and y2 represents STY observed at starting and ending of each test. yX represents STY estimated at the beginning of each respective run except run No. 6 and 7. Run Catalyst Lfl? No. volume (m”)
React. temp. (“C)
SV (l/h)
MR
Replacement Test STY (ton/day*m3) rate by new period catalyst (day) Starting Ending Estimation (l/day) (Yl) (Y2) (YX)
38.4 39.7 38.6 36.1 33.8
1.29 1.34 1.29 1.23 1.19
170 172 175 177.5 180
142 137 142 152 162
2.5 2.5 2.5 2.5 2.5
-
6 15 27 18 8
1.113 1.185 1.338 1.358 1.425
1.104 1.149 1.251 1.281 1.370
1.11 1.19 1.39 1.50 1.67
55.7 55.9 56.2
1.83 1.83 1.83
174 174 174
100 100 100
2.5 3.0 3.5
0.0027 0.0027 0.0027
12 12 5
0.980 1.002 0.993
0.948 0.920 0.920
0.98 1.03 1.12
57.0 57.0 57.0 56.5 55.7 55.0 54.0
1.87 1.87 1.87 1.85 1.83 1.81 1.75
177.5 180 182.5 185 187.5 190 192.5
100 100 100 102 104 105 108
2.5 2.5 2.5 2.5 2.5 2.5 2.5
-
7 8 11 11 7 5 8
0.886 0.950 0.983 1.011 0.983 1.010 1.032
0.861 0889 0.912 0.914 0.927 0.956 0.934
0.89 0.98 1.09 1.21 1.30 1.40 1.51
64.0 57.0 52.0 49.0
2.09 1.84 1.73 1.63
174 174 174 174
100 100 100 100
2.5 2.5 2.5 2.5
-
8 11 12 6
0.950 0.967 0.942 0.903
0.926 0.935 0.900 0.892
0.95 0.99 1.00 1.01
50.0 49.0 47.5 45.5
1.66 1.63 1.59 1.52
174 174 174 174
100 120 140 100
2.5 2.5 2.5 2.5
-
9 10 13 9
1.028 1.037 1.043 0.985
0.986 1.014 1.024 0.958
1.03 1.08 1.11 1.06
61.0 55.0
2.00 1.81
174 174
100 100
2.5 2.5
0.0137 0.0103
14 14
0.891 0.934
0.918 0.933
0.92 (y,) 0.93 (Yl)
47.0 43.5
1.57 1.45
174 174
120 120
2.5 2.5
-
6 11
1.229 1.125
1.206 1.105
1.21 1.13
-
Therefore, it is assumed that the STY of 0~0 the observed yet 0 days later as follows: In %B
-Y&2
YOA =
fYOBhB)
= In YOB XYI3C
In YOA =YOC
(yoC) can be estimated
(Yz) (Yl)
from
(5)
78
The estimation method at the same time and same activity level obtained from the STY ratio around the change of reaction conditions described by eqn. ( 5) is confirmed and considered applicable to other results of this series of operational tests. These results are shown in Table 1. CONCLUSION
In order to investigate the optimum fluidizing conditions and reaction conditions of the fluidized reactor for the synthesis of vinyl acetate from acetylene, the method of analysis of the operational data obtained using a large industrial fluidized reactor was studied. For utilization of the operational data, it is considered necessary to establish the estimation method of the STY indicated by the mixed catalyst when catalysts of different catalytic activity are additionally supplied and also the estimation method of the STY at the same time and same activity level. With regard to these points, from In (STY) -time plots, utilizing the fact that the deterioration rate of the catalyst activity is proportional to the STY, a rise in STY corresponding to an additional supply ratio of more active catalysts was observed. The additivity of the STY was confirmed. Also, using a ratio of the STY around the change of reaction conditions, the estimation formula for the STY at the same time and same activity level was determined. At the same time and same activity level, the respective STY values corresponding to the respective reaction conditions were estimated and obtained. This provides useful basic data for studying optimum reaction and fluidizing conditions. Reports describing the results of optimum reaction and fluidizing conditions using these data will be published shortly [ 12,131.
ACKNOWLEDGEMENTS
The authors are grateful to the staff of the manufacturing section of the Nakajyo plant of Kuraray Co., Ltd. for operating the equipment to provide them with the operational data for this report, as well as the management of the company for permitting us to use this data. Also, we would like to thank Professor Yichi Murakami of Nagoya University for giving advice and instruction for the preparation of this report. REFERENCES
1 2 3
T. Kawaguchi, H. Kimura and T. Wakasugi, Sekiyu Gakkaishi, 29 (1986 ) 168. A. Mitsutani, Shyokubai (CatalystO, 4 (1962) 127. A. Mitsutani and T. Kominami, J. Chem. Sot. Jpn. Pure Chem. Sec., 80 (1959) 886.
79
A. Mitsutani and T. Kominami, J. Chem. Sot. Jpn. Pure Chem. Sec., 80 (1959) 893. A. Mitsutani and T. Kominami, J. Chem. Sot. Jpn. Pure Chem. Sec., 80 (1959) 890. A. Mitsutani and T. Kominami, J. Chem. Sot. Jpn. Pure Chem. Sec., 80 (1959) 895. S. Yano and S. Matsumoto, J. Chem. Sot. Jpn. Ind. Chem. Sec., 54 (1952) 498. T. Kawaguchi, G. Hira, T. Wakasugi and T. Tatsumi, Kagaku Kogaku Ronbunshu, 11 (1985) 708. 9 T. Kawaguchi and T. Wakasugi, in preparation. 10 T. Kawaguchi, J. Nakagawa and T. Wakasugi, Appl. Catal., 32 (1987) 23. 11 T. Kawaguchi and T. Wakasugi, Kagaku Kogaku Ronbunshu, 12 (1986) 499. 12 T. Kawaguchi, T. Wakasugi and T. Wakasugi and N. Hatano, KagakuKogaku Ronbunshu, 12 (1986) 133. 13 T. Kawaguchi and T. Wakasugi, Kagaku Kogaku Ronbunshu, 12 (1986) 627.