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Ind. Eng. Chem. Res. 1996, 35, 2570-2575
Comparison of the Technology of Oxidative Dehydrogenation in a Fluidized-Bed Reactor with Those of Other Reactors for Butadiene Wu Xingan* and Liu Huiqin Department of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, People’s Republic of China
This paper describes a comparison among the reactor technologies used in the process of oxidation. For dehydrogenation of butene into butadiene, three reactor types are compared: (1) a fluidized-bed reactor using multi revolving link stoppers with a group VIII variable-valence catalyst, (2) an adiabatic fixed-bed reactor, and (3) a polytube, constant-temperature fixed-bed reactor. The results of the comparison indicate that the polytube fixed-bed reactor at constant temperature is better than the fluidized-bed reactor, which in turn is better than the adiabatic reactor. Using a polytube, constant-temperature fixed-bed reactor with butene’s space velocity of 400 h-1, butadiene yield reached 78.7%, butene conversion reached 86.1%, and butadiene selectivity reached 91.4%. If these results can be achieved in industry, they will be the world records. Introduction
Main Reaction CH3CH2CH
Butadiene is the principal monomer used for the production of synthetic rubber. In 1994 the world production capacity was 13.166 million ton/yr, and 80% of the synthetic rubber was made from the raw material butadiene.1 The oxidative dehydrogenation of butene into butadiene is one of the major process routes. Having solved the catalytic problems, the key research work has been concerned with the reactor. In the process of the oxidative dehydrogenation of butene into butadiene, there are many kinds of catalysts, one of which is ferrite.2-6 Use of the ferrite catalyst, with high catalytic activity and fewer byproducts, has been progressing rapidly and attracting much interest.7-11 In the procedure there are three reactors being used to produce butadiene.7 The first one is an adiabatic fixedbed reactor, being used in many petrochemical companies.8 The second is a fluidized-bed7 reactor and the third a fixed-bed reactor of the constant-temperature polytube type.7 We have studied the technologies involved and have had much experience with these reactors. We have also used the fluidized-bed reactor for the oxidative dehydrogenation of butene into butadiene in small-scale and pilot-scale tests over a group VIII variable-valence catalyst, which is the only known work of this type. We discovered that the oxidative dehydrogenation of butene in the fixed-bed reactor of the constant-temperature polytube type exhibited high efficiency over a group VIII variable-valence catalyst. This paper will report on the reactor procedure in the fluidized bed. On the basis of our research results, the engineering technologies and economies of the three reactors will be compared and discussed.
Chemical Reaction and Final Kinetic Model The reactions taking place are as follows:
CH2
1-butene or CH3 CH
CH CH3
370 ± 10 °C 1 trans-2-butene + O catalyst 2 2
CH2
CH
or CH
CH2
1,3-butadiene
CH
CH3
+ H2O
CH
CH3
cis-2-butene ∆H720K° = –125.4 kJ/mol Byproduct Reaction C4H8 + 4O2
4CO + 4H2O
∆H720K° = –1268.54 kJ/mol
C4H8 + 6O2
4CO2 + 4H2O
∆H 720K° = –2524.72 kJ/mol
C4H8 +
1 O 2 2
+ 2H2O
∆H720K° = –250.8 kJ/mol
O
On the fixed bed in 315-430 °C, the final kinetic model is3,12
rBD ) 6.328 × 108e-19.6/RTPu0.9Po0.1 Pu ) butene partial pressures, atm Po ) oxygen partial pressures, atm Experimental Section Pilot Process and Reactor Construction. The Oxo-D pilot process flow diagram is shown in Figure 1. The multi revolving link stopper fluidized-bed reactor is 800 mm in diameter and contains catalyst between 450 and 550 kg in weight. The reactor consists of three parts. The bottom and middle are the reactive parts, while no reaction occurs
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Ind. Eng. Chem. Res., Vol. 35, No. 8, 1996 2571
Figure 1. Oxo-D pilot process flow diagram. Table 1. Butene Feedstock Componenta wt % component
20 mm diameter small reactor
800 mm diameter pilot plant
1-butene trans-2-butene cis-2-butene C40 C5 total
13.0 59.8 27.1 0 0.0 99.9
0.6 43.6 32.9 22.2 0.71 100.0
a Catalyst is a group VIII variable-valence catalyst in small and pilot-plant reactors.
in the top part. The bottom and middle reactive parts have an internal tubular heat-exchange mechanism. The bottom reactive part has three groups of fountain sprays; each group has two spray heads, and they are evenly placed to spray deionized water to control the temperature of the catalyst. The oxidative dehydrogenation process involves vaporized butene and water steam being mixed in a preheater between 130 and 180 °C. Air and the butene/ steam mixture are then mixed, in a mixer, prior to entering the fluidized-bed reactor from the bottom. This mixture reacts with the catalyst and produces butadiene gas, which is channeled through the top of the reactor to a water washing tower, where the catalyst powder is separated from the gas produced using a two-tier graded circle wind separator. When the butadiene gas enters the water washing tower, it is cooled and the catalyst and oxidized impurities are dissolved in water. The resulting butadiene gas is compressed to 1.3 MPa and is suitable for usage in industrial production processes. In addition, there is a cone wind cup gas distributing board which distributes the gas through perforated holes at a rate of about 1.4%. The multi revolving link stoppers of the reactor increase the contact between the catalyst and the reaction gas, improving the conversion and selectivity rate, while the direction of the multi revolving link stoppers reduces the mixture of catalytic powder with the butadiene gas which is produced.
Butene Feedstock and Catalyst. See Table 1. Analytic Method. Gas chromatography is used for the analysis of the butene feedstock in the effluent.12 All hydrocarbons from C1 through C4 as well as CO, CO2, O2, and N2 are detected and measured. Analyses are made for acetylenes and carbonyls including TOC and TC in the condensation process to determine the equilibrium of feedstock and effluent. The data are fed into a computer along with calculated butadiene selectivity, butene conversion, and butadiene yield. Small-Sized Reactors. The small-sized fluidizedbed reactor is a quartz tube measuring 22 × 1.5 × 1000 mm. It contains 24 Storey baffles, each distance of them is 20 mm. The area of each section is 2.835 cm2. The small fixed-bed reactor has a stainless steel tube measuring 16 × 1.0 × 300 mm, with porcelain rings and fiberglass 128 mm above and below the catalyst. The area of the section is 1.5 cm2 and requires a constant temperature. Results and Discussion For results obtained with the catalyst on a small-sized fixed-bed reactor and a small-sized fluidized-bed reactor, see Tables 2 and 3. From the reaction results, we can see that when butene’s space velocity is 400 h-1, in a small-sized fixedbed reactor butadiene yield can reach 78.7%, butene conversion can reach 86.1%, and butadiene selectivity can reach 91.4%. These are very good results. In a small-sized fluidized-bed reactor, where butene’s space velocity is 400 h-1, the best butadiene yield is 66.4% and butene conversion is 72.8%. Butadiene selectivity is 91.2%. These results are less favorable than those obtained in the fixed-bed reactor. Explanations for this difference are discussed below. In the small-sized fixed-bed reactor, the gas and catalyst solid’s contact rate is high and the reacting material resembles a piston flow with no return flow; also the reaction temperature is easy to control within a constant range. The apparatus can thus be viewed as very similar to a constant-temperature tube fixedbed reactor. Therefore, butene conversion, butadiene selectivity, and butadiene yield are all high. However, a constant-temperature fixed bed must have plenty of linear tubes and a transition heat system, the catalyst must be a small ball, and the technology is difficult and expensive. The small-sized fluidized bed has a fluidized bed’s character. The space between the gas and catalyst solid is large and the contact rate is low. Though butene space velocity is the same as in the former case, butene conversion is (86.1 - 72.8) 13.3% lower. The 24 Storey baffles in the fluidized-bed reactor reduce the reaction material’s return, giving the appearance of a piston flow on the whole. There is a partial return between the baffles, but it is not much. The condition of constant temperature is better than that in
Table 2. Small-Sized Fixed Bed with Different Space Velocity Experimental Results space velocity (m3/m3‚h-1), h-1
reaction temp, °C
C24 :O2:H2O, mol
reaction P, MPa
butadiene yield, mol %
butene conversion, mol %
butadiene selectivity, mol %
CO, mol %
CO2, mol %
400 500 600 700
338 325 332 327
1:0.84:15 1:0.8:15 1:0.8:15 1:0.8:15
0.1 0.1 0.1 0.1
78.7 67.0 63.0 59.2
86.1 76.0 74.0 69.7
91.4 85.0 83.1 85.5
2.9 8.0 5.4 6.5
4.6 4.0 3.5 3.6
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2572 Ind. Eng. Chem. Res., Vol. 35, No. 8, 1996 Table 3. Small-Sized Fluidized Bed with Different Lot Catalyst Experimental Results
reaction results, % Cat. No.
Y
C
S
983-I 983-II 983-III 983-22 983-23
63.0 57.5 55.0 62.2 66.4
67.7 65.3 64.0 67.4 72.8
93.0 88.0 86.0 90.8 91.2
CO CO2 1.4 2.2 2.6 1.6 2.0
assessable requirement of small fluidized bed
3.3 T reaction ) 340-360 °C 5.6 space velocity of butene ) 400 h-1 6.4 C2-:O2:H2O (mol) ) 1:0.68:12 4 3.6 4.4
the adiabatic fixedbed, so butadiene selectivity can reach a high level of 91.2% and butadiene yield can reach 66.4%. These are good results. Because the structure of a fluidized-bed reactor is simple and its price is low and because the butadiene yield increased about 16% compared with the original 48-51% of the Mo family catalyst industrial fluidized-bed reactor, we believe that a fluidized-bed reactor and a group VIII catalyst form the basis of a readily applicable industrial process. Investigation of the Catalyst Magnifying Effect. Using the same catalyst and reaction conditions, the typical reaction data for oxidative dehydrogenation of butene in a 20 mm diameter small-sized fluidized bed and in a 800 mm diameter middle-sized fluidized bed are shown in Table 4. From data in Table 4, though the scale-up factor of the reactor’s section area is 1600, the scale-up factor of the catalyst’s loading capacity is 3762.5. On a directive baffled fluidized bed though, the magnifying effect is small, and the result of the reaction is very satisfactory because of the good dispersing quality of the gas and solid contact in a fluidized bed and the constanttemperature quality of the reactor. Investigation of the Effect on the Quality of Long-Term Usage of Catalyst. The group VIII catalyst was effective in the 800 mm fluidized-bed reactor for 1372 h and duplicated the results generated in the laboratory. The butadiene yield reached 60-63% and a selectivity of about 90%; the content of the oxygenated compounds was about 0.6%. Condensation product and water drained off from the water washing tower appeared neutral; there was no organic acid produced, and the content of acetylenic byproducts in production gas was 20-30 ppm. The average friction loss of catalyst that had been converted was 2.91 kg/ton of butadiene, which is small, and it appeared that the catalyst was slowly renewing in the fixed bed. Influence of Operation Variables. Influence of the Reaction Temperature. Within the range 368378 °C, the highest yield and selectivity were achieved. Figure 2 shows that, in the temperature range 360380 °C, yield and conversion increase with the temperature’s increase, while selectivity has a slight tendency to decrease, CO2 production increases slightly, and CO and Oxoc production tends to remain static.
Figure 2. Influence of the reaction temperature on reaction results.
Figure 3. Influence of the oxygen to butene ratio on reaction results.
Influence of the Oxygen to Butene Ratio. In the industrial process, changing the oxygen to butene ratio affects the reaction process. Increasing the amount of oxygen in the O2/C2ratio, even up to 1.0/1.0, will 4 increase the strength of the reaction process. Accordingly, when the reaction is strong, controlling the reaction and increasing selectivity can be achieved through the reduction of oxygen. Figure 3 shows that yield and conversion are increased as the ratio of oxygen to butene is increased, within the range of an oxygen to butene ratio of 0.550.80, although the selectivity decreases slightly. An oxygen to butene ratio of 0.65-0.70 is preferred. Influence of the Water to Butene Ratio. Within the range of a water to butene ratio of 8-10.5, the oxygenated compound yield decreases slightly (Figure 4), but the yield, conversion, and selectivity do not change as the water to butene ratio increases. The H2O/C24 ratio can be changed to regulate the temperature; therefore, when the reaction temperature is high, increasing the quantity of steam or water will reduce the temperature and decreasing the quantity of steam or water will have the opposite effect. The regulation of steam or water has a 2-fold effect: first, it can influence the catalyst’s regeneration through the reaction of the carbon on the catalyst (H2O + C ∆f
Table 4. Magnifying Effect of Oxidative Dehydrogenation of Butene in the Fluidized-Bed Reactora reaction results, % reactor diameter, mm
catalyst loading wt, kg
butadiene yield
butene conversion
butadiene selectivity
CO + CO2 byproduct
oxygenated byproduct
20 800
0.1198 450.0
61.5 62.3
68.3 69.3
90.0 90.0
6.2 6.4
0.6 0.6
a
2Reaction temperature, 360-370 °C; butene space velocity, 300 h-1; O2/C24 ) 0.88; H2O/C4 ) 9-11 (mol).
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Ind. Eng. Chem. Res., Vol. 35, No. 8, 1996 2573 Table 5. Typical Reaction Conditions and Results in a 800 mm Diameter Fluidized Bed
Figure 4. Influence of the water to butene ratio on reaction results.
Figure 5. Influence of the butene space velocity on reaction results.
hours (together) program butene, L/h (liquid) air, m3/h H2O, kg/h (steam) O2/C24 , mol H2O/C24 catalyst, kg bed temp, °C bed pressure, MPa space velocity, h-1 line velocity, M/s butene/feedstock, % yield (BD), % conversion, % selectivity, % remains O2, % CO, % CO2, %
250 0:30′ 518 307 750 0.705 10.22 450 370 0.123 288 1.50 73.8 61.8 68.3 90.5 0.4 1.7 4.8
258 8:30′ 518 307 750 0.706 10.2 450 371 0.121 288 1.57 73.8 62.0 68.2 91.0 0.8 1.6 4.6
266 16:30′ 518 307 765 0.672 9.95 450 375 0.1237 300 1.53 77.1 62.0 69.2 89.6 0.4 1.9 5.3
tene space velocity is increased. Figure 5 shows that conversion and selectivity are best obtained when the space velocity is in the range 250-300 h-1. Increasing O2/C24 and the temperature will cause the space velocity to rise higher than 400 h-1. Influence of the Reactor Inlet Pressure. Because of the existing pressure differences among the processes in the reactor, there is 0.03-0.07 MPa in each part of the reactor. The oxidative dehydrogenation of butene is a reaction of volume expansion; therefore, with an increase in the reaction pressure, the conversion rate declines. cat.
CH3CH2CHdCH2 + 1/2O2 9 8 ∆ cat.
CH2dCHCHdCH2 + (H2 + 1/2O2) 9 8 ∆ CH2dCHCHdCH2 + H2O
Figure 6. Influence of the reactor inlet pressure on reaction results.
Figure 6 shows a conversion decline of 5% when reactor inlet pressure is increased from 0.035 to 0.1 MPa. The reactor does not influence selectivity. Influence of the Butene Feedstock’s Purity. The content of butene in the feedstock changes within the range 77-97%, under favorable reaction conditions, and has no influence on the reaction results. The impurity of the feedstock is C40 not iso-C42- (see Figure 7). For typical reaction conditions and results, see Tables 5 and 6. Comparison of Butadiene Processes Energy and Raw Material Requirements. The processes available for the production of primary butadiene, the data from published sources detailing conversion and per pass yields, and steam and feedstock requirements are given in Tables 7 and 8. Conclusion
Figure 7. Influence of the butene feedstock’s purity on the reaction.
CO + H2); second, it can act as a safety measure during the oxidative dehydrogenation process. Influence of the Butene Space Velocity. Butadiene yield and butene conversion decline as the bu-
1. Oxo-D fixed-bed adiabatic reactor has a simple construction and permits easy loading of the catalyst. Butene conversion reached 65%, butadiene conversion 60%, and selectivity 92%. These results are all better than those for catalytic dehydrogenation of butene and the original catalytic oxidative dehydrogenation of butene, representing an advance in technology although the fixed-bed adiabatic reactor does not have the ability to change heat.13 A gas fuel heating furnace is required
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2574 Ind. Eng. Chem. Res., Vol. 35, No. 8, 1996 Table 6. Oxygenated Compound Distribution in a 800 mm Diameter Fluidized Bed oxygenated compound distribution, % hours (together)
byproduct, % butene
furan
acetaldehyde
1400
0.35
25.7
11.4
acrylic aldehyde
acetone
trace
48.6
methyl acrylic aldehyde
toluene
trace
trace
benzene 14.3
methylacetone trace
butanyl ketone
all together
1.0
100.0
Table 7. Comparison of Several Processes for Making Butadiene process catalysis dehydrogenation I8 catalysis dehydrogenation II8 Oxo-D fixed-bed8 adiabatic Oxo-D fluidized-bed constant-temp tubule reactor Oxo-D (small test)
conversion % per pass
yield % per pass
mol of BD produced/ mol of butene converted
mol of steam fed/ mol of BD produced
25
16
0.65
62.5
40
34
0.85
87.0
65
60
0.93
20.0
68.3 86.1
62.2 78.7
0.925 0.935
16.0 15.0
Table 8. Comparison of the Technology of Oxo-D in Three Reactors for Butadiene
reaction temp °C butene space velocity, h-1 O2/C24 , mol H2O/C24 , mol reaction pressure, MPa butadiene yield, % butadiene selectivity, % oxo compounds, % organic acid, % alkynes, %
fixed-bed,8 adiabatic
fluidized-bed, many revolving stoppers
constant-temp tubule reactor (small test)
inlet, 320 °C; outlet, 600 °C 300 0.55-0.60 11-16 0.18-0.25 60-63 92 0.7 0.08 0.259
inlet, 146 °C; outlet, 370 °C 250-350 0.65-0.80 8.5-11 0.135-0.20 60-63 90-91 0.3-0.1 0.00 0.008-0.009
inlet, 330 °C; outlet, 340 °C 400 0.84 14 0.18-0.25 78.7 91.4 0.3-0.4 0.01 0.008-0.01
to produce an inlet temperature of 320 °C. The temperature difference of the reactor can reach 280 °C and the steam to butene ratio 11-16, which requires a large quantity of steam for loading heat, which, in turn, increases production costs. It also makes the production rate of oxygenated compounds, organic acids, and alkynes higher while complicating the refinement of butadiene. 2. The structure of the fluidized-bed reactor also is not very complex and is inexpensive and easy to produce. Butene conversion reached 68.3%, butadiene yield reached 62.2%, and selectivity reached 90-91%, results about the same as those for the Oxo-D fixedbed reactor. However, the reactor inlet temperature is 140-150 °C and does not require a gas fuel heating furnace to begin the reaction. The reactor bed is held at a constant temperature of about 370 °C, and the steam to butene ratio, which is 8.5-11, is low. This lowers production costs by saving on the production of steam and gas fuel. The small change in the reactor temperature reduces the production of oxygenated compounds, organic acids, and alkynes and reduces the difficulty in refining butadiene. This lowers production costs too. Compared to the technology of a fixed-bed adiabatic reactor, the technology of the fluidized-bed reactor is an improvement. 3. The results of the 14 mm diameter small-sized fixed-bed reactor show that, at a constant temperature and using the same group VIII catalyst with a butene space velocity of 400 h-1, butene conversion can reach 86.1%, butadiene yield can reach 78.7%, and selectivity can reach 91% per pass. From a chemical engineering viewpoint, the constanttemperature fixed-bed reactor is the best reactor. If this reactor process can be developed as an industrial
production process that exchanges heat and does not require a gas fuel heating furnace, increases per pass yield of butadiene and reduces the difficulty in butadiene refinement, and increases space velocity to increase plant capability, therefore reducing production costs, it will increase the production of butadiene to higher levels than currently being achieved. Literature Cited (1) Int. Rubber Dig. 1994, 47 (11), 4. (2) Rennard, J. R.; Innes, A. R.; Swift, E. H. Oxidation Over MgCrFeO4 and ZnCrFeO4 Catalysts. J. Catal. 1973, 30, 128138. (3) Sterrett, J. S.; Wcllvried, H. G. Kinetics of the Oxidative Dehydrogenation of Butene to Butadiene Over a Ferrite Catalyst. Ind. Eng. Chem. Process. Des. Dev. 1974, 13 (1), 54-59. (4) Qiu, F.-Y.; Weng, L.-T.; Ruiz, P.; Delmon, Effect of Antimong(IV) Oxide, Bismuth Phosphate and Tin(IV) Oxide on the Catalytic Properties of Compound Oxide Catalysts in the Oxidative Dehydrogenation of n-Butene. Appl. Catal., 1989, 47, 115123. (5) Qiu, F.-Y.; Weng, L.-T.; Sham, E.; Ruiz, P.; Delmon, Effect of Added Sb2O4, BiPO4 or SnO2 on the Catalytic Properties of ZnFe2O4 in the Oxidative Dehydrogenation of Butene to Butadiene. Appl. Catal. 1989, 51, 235-253. (6) Liaw, B. J.; Cheng, D. S.; Yang, B. L. Oxidative Dehydrogenation of 1-Butene on Iron Oxyhydroxides and Hydrated Iron Oxides. J. Catal. 1989, 118, 312-326. (7) Wu, X.; Liu, H. Reactor for Oxidative Dehydrogenation of Butene into Butadiene. J. Chem. Eng. Chin. Univ. 1995, 4, 370378. (8) Welch, M. L.; Croce, J. L.; Chrisimann, F. H. Butadiene Via Oxidative Dehydrogenation. Hydrocarbon Process. 1978, Nov, 131-136. (9) Liu, G. F-90 Ferrite Fixed Bed Catalyst for Oxidation Dehydrogenation of Butene into Butadiene. China Synth. Rubber Ind. 1991, 74 (2), 179-182.
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Ind. Eng. Chem. Res., Vol. 35, No. 8, 1996 2575 (10) Chen, A.; Runchou, W.; Xinhui, H.; Jianxin, M.; Ren, W.; A New Technology of Feeding Oxygen Through Different Inlets for Oxidative Dehydrogenation of Butene B90 Catalyst. (China) Petrochem. Technol. 1995, 24 (1), 1-3. (11) Chen, A.; Runchou, W.; Xinhui, H.; Qian, S.; Shengyong,Y. Isothermal Fixed Reactor for No-Chromium-Ferrite B90 Catalyst. (China) Petrochem. Technol. 1995, 24 (2), 79-81. (12) Zhou, W.; et al. Kinetics of the Oxidative Dehydrogenation of Butene to Butadiene Over a H-198 Catalyst. Lanzhou Institute of Chemical Physics. Academia Sinica, Report 1 Oct 1983.
(13) Wu, X.; Wang, C. Development of Catalyst SnPLi for Oxidative Dehydrogenation of Butene into Butadiene in Pilot reactor. China Synth. Rubber Ind. 1979, 3, 201-206.
Received for review June 9, 1995 Accepted April 16, 1996X IE950347O X Abstract published in Advance ACS Abstracts, July 1, 1996.
