Kinetics of Polymerization of Olefins in Production of Polymer Gasoline Paul Friedman and Kenneth 1. Pinder' Department of Chemical Engineering, University of British Columbia, Vancouver 8 , B.C., Canada
A kinetic study was made of the polymerization of a mixture of propene and butene isomers to their dimerized products. A commercial solid phosphoric acid catalyst was used in a packed-bed flow reactor. The temperature was varied from 300" to 460°F a t a constant pressure of 515 psia. The olefin conversions were measured a t different flow rates and the data were fitted to an empirical model. The activation energy for the reaction was 7540 cal/g-mol. Gas film resistance effects were not observed, but pore diffusion resistance was present. Some qualitative results on the relative reaction rates of different olefins were also reported.
T h e commercial process for the catalytic polynierizatioii of olefins t,o a liquid hydrocarboii fraction which boils within the gasoliiie range was developed in 1931-5 (Ipatieff, 1934, 1935). Eve11 though niaiiy polygas plaiits have been built. by use of the ratalytic polynierizatioii process developed by the U i i i vcr.~nlOil I'roducts Co., little has been reported in the literature 011 t l i p kinctics of the react'ion on their solid phosphoric acid cat5alyst. Several niechaiiisnis have been proposed for the catalytic polytnerizat8ioii of olefiiis. Whitminoreproposed that a carlioiiium io11be added to an olefin to form a higlier-niolecularweight carboiiiuni ion n-hich would then yield the olefin polyniei' 1))- eliiniiiat~ioii of a protoii (Farkas and Farkas, 1942; Pchiiwliiig and Iliatieff, 1950). Ipatieff (1935) postulated that tlip polj~nierizationof olefiiis in the presence of a phosphoric acid catalyst involves the interaction of two molecules of phos~horicacid est,er. Oiie of the feiv papers on the industrial process was written by Sliaiiley and Egloff (1939) who report'ed on the effect of cntalyst lied teinperature on olefin conversion. Lnnglois n i i d Kalkey (1951) studied the effect' of teniperat w e , phnsl~horicacid conceiitratmioii,quartz particle size, aiid olcfiii conceut,i,atioii on t,he rate of the polymerixatioii reactioii. Since their work was carried out, Jyith a catalyst of liquid ~11iosplirnicacid on quartz, doubts have been raised (Egloff mid V-eltiert, 1951) as to wliet8hertheir results may be applied t,o the 1701' process which uses a solid catalyst of phosphoric a c d on n kicselgulir support'. 'l'hr purlioae of this work was to obtain a rate equat,ion wliicli would describe the polynierizat'ioii of an industrial inist,urc of olefins oii a coniinerciallq- available solid phosplioik acid catalyst. The act.unl mechanism of the reaction \vas not considered. Experimental
Apparatus. A scheiiiat,ic flow diagram of t h e esperimciital equipment is show1 in Figure 1. 'l'lir feed M-:E ot)t8ainedfrom a nearby refinery and is typical of thc feeds to such industrial units (Table I). The feed cylinder, ~ l i i c hwa,5 filled originally to about 80y0 of its volume, x i s held in a i inverted posit,ioii to allow the withdrawal of a liquid it,renm of alniost constant coinposition. T o increase the 1
T o whom correspoiideiice should be addressed.
548
Ind. Eng. Chem. Process Der. Develop., Vol. 10, No. 4, 1971
feed pressure, the cylinder was heated by an external coppershielded electric heater manually controlled by a Variac. T o prevent flashing iii the metering pump, the feed was cooled in two steps, once before the needle valve control a n d rotameter a n d again after the point of niixiiig with propane. Propane could be used as a feed d i l ~ e i i tThe ~ . control of the propane flow was similar to that of the feed. A controlled volume pump was used to force the niised feed through two heaters aiid the reactor. The heaters mere made of st'aiiiless steel pipe 1-in. in diameter and 16-in. long. The esit temperat,ures of the feed were measured with iroiicoiist'aiit.aii thermocouples a n d cont'rolled by varying the voltage to a 500-W immersion heater in each vessel by means of a Variac. The reactor was a stainless steel pipe, 2-in. in diameter aiid 12411. long, flaged a t each end. The length of the reactor was divided into sections by screens held in position by spacers. The first, sect'ion served as a final heater. Even heat distribution was obtained by a packing of l/i-in. copper rings. The catalyst was supported between screens in the last three sections of the reactor. Six thermocouple inlets were spaced along the reactor so shielded thermocouples could be inserted into the center of the gas space between the catalyst beds. Good teinperature control within the reactor was obtained by use of a proport.iona1 plus reset controller to adjust the iiiput to a shielded heating cable which was wound under the insulation of the reactor. The controller was connected to the iron-constantan t,herrnocouple located at the entrance to the catalyst, section. The obher thermocouples were read a t regular t,ime intervals with a manual potentioineter. X coiistant pressure (515 psia) was held in the system by a back pressure control valve and a coiitroller with proportional plus reset modes. The gases leaving the reactor passed through this valve to a purge line. Product samples mere obtained directly after the reactor through a sample line connected to a gas chromat,ograph. The sanililes were cooled to coiideiise the gases, then measured directly into the iiist,rument by a high pressure liquid sampling valve. The gas chromatograph output was integrated directly on the recorder. Procedure
At the beginning of each day's runs, the reactor was filled a i t h new catalyst. The system was then filled with liquid feed
CHROMATOGRAPH
AND RECORDER
Figure 1. Flow diagram of experimental apparatus
a t the cylinder ixesqure. Then the preheaters and reactor heater nere turned on. When the deqired temperature a as reached, the metering pump was stai ted and the flow rate was adjusted by means of the metering sciew on the pump. IVhen mixtures of propane and feed were used, their ratio was set by adjusting the needle valves located before the rotameters Once the temperature inside the reactor reached steady state, sampleq of the feed and product were fed to the gas chromatograph. Conditions were then adjusted for the nevt iun. 'The dew point foi a typical feed is about 250'F. Thus, the feed mas vaporized completely before eiiteiiiig the catalyst section. Steady state in the reactor \\as shonn by coiistant values of tempei ature on all the reactor thermocouples. Xormallg. about one hr 1Tas needed to reach steady state. RUILS were made a t 300°, 340°, 380°, 420°, and 460°F and a t a series of feed flow iates while the feed concentration and the volume and particle size of catalyst c ere kept constant. Another set of Iuns was made t o check the possibility that the gas film diffusion rate was contiolling. Feed f l o n rates were ~ a i i e dbut the space-time, V F, was maintained constant. Runs were made a t various temperature lelels and fixed particle size. A final series of runs was made to check for the effect of pore diffusion. I n thiy case the temperature, the flow rate, and the
composition were maintained constant while two diff ereiit size fractions of catalyst particles were used; 4.76-6.iB nim and 0.420-0.595 mm. Results
The total olefin conversions, obtained experimentally a t different olefin flow rates and temperatures are shown in Figure 2. A n integral analysis was used to determine a rate expression which would satisfactorily fit the data. A rate expression \vas selected and inserted into the plug flow reactor equation.
V
=l$
(1)
The right-hand side of Equation 1 was integrated iiumerically for each experimental run and a linearity test was then performed to check the selected rate equations. The empirical rate equation of Langlois and Ralkey (1951) as well as a first-order and a second-order rate equat,ion was tested. Figure 3 shows t h a t none of these three equations fits the data well. An empirical equation was then developed which gave a reasonably good fit (Figure 4).
It was felt that a more sophisticated procedure of model Table 1.
Range of Feed Compositions
Olefin
Ethane Propane Propene Isobutane n-butane Isobutene 1-butene trans-2-butene cis-2-butene Isopentane n-Pentane I-Pentene 2-Methyl-1-butene trans-2-pentene 2-llethyl-2-butene Total olefins
+
Mole
70
0 1-0 9 2 1-24 14-20 23-26 10 5-20 6-7 2 4 2-5 8 2 8-4 3
h
g05I
460'F
3 3-7 2 0 1-0 2 0 1-0 2
0 2-0 3 0 1-0 2 0 1-0 2 28-36
420.F
380.F 340'F
3OODF O f t
0 IO
20
30 40 50 60 70 BO FLOW RATE OF OLEFINS F I M O L E W H R I
90
100
Figure 2. Effect of olefin flow rate and temperature on olefin conversion Ind. Eng. Chem. Process Des. Develop., Vol. 10, No.
4, 1971 549
Table
Olefin flow rate, mol/hr
1
2.29 1.90 1.52 1.14 0.76 0.38
8.20 10.74 13.96 15.70 21.30 31.59
II.
30.27 32.61 ...
38.87 ...
49.78
Olefin Conversion as a Function of Particle Size Catalyst particle size: L = 4.76-6.73 m m S = 0.420-0.595 mm
11.oo 13.00 15.50 19.00 25.76 36.68
3.69 3.03 ... 2.47 ... 1.58
developnieiit and discrimiiiatioii was not justified in this work for two reasons: The basic goal of this study was a rate equation which would adequately describe the results from an iiidustrial reactor rather than one which would be used in a mechanistic study. Statistical differentiation between more complex models would probably fall within the limits of the experimental error. The rate coiistaiit defined by Equation 2 was calculated by making a least-squares fit of the data to that equation. h n hrrheiiius plot of the rate constants a t five temperatures (Figure 5) was made and the best straight line was fitted by t,he least-squares method. The equat,ioii of this line is:
_ 7640 _
2 . 8 7 x 105e R T (3) The value of 7540 cal/g-mol is close to the value of 7920 call g-mol reported by Langlois and Walkey (1951) for a liquid phosphoric acid on quartz catalyst system. Their Arrhenius plot is also shown on Figure 5. k
=
Mass Transfer Resistance
To check whether mass transfer to t,he catalyst was effect,ive in coiitrolling the rate of reaction, we niade runs a t a series of flow rat.es holding the rat,io V I F constant. We found the conversion was independent of flow rate; hence, x e assume that under the conditions of these experiments, gas film diffusion is unimportant. Pore Diffusion
The runs which were made in the initial studies used the commercial catalyst screen fraction bet'ween 4.76 and 6.73 Inm. This fraction contained the major part of the catalyst as used in iiidustrial reactors. T o test the possibihy t'hat pore diffusion was an important factor, we carried out a number of runs with a screen fract,ioiibetween 0.420 and 0.595 mm. If pore diffusion is negligible, the rate of reaction should be
30.79 33.56 39.07 43,78 50,40 55.06
2.79 2.58 2.53 2.31 1.96 1.51
25.06 28,23 33.05 35.89 45.73 52.35
.
I
.
76.68 77.00 79.99 87.48
... 2.71 2.32 2.22 1.92
the same for both sizes of catalyst particles. For strong pore diffusion control, the rate of reaction varies inversely as the particle size (Levenspiel, 1962). Table I1 shows that pore diffusion does affect. the rate of reaction. The rat,io of conversions varied from 1.5 to 3.7 for a particle size ratio of about 11. For the small particles, over the range of flow rates used in these tests, the Reynolds number varied from about 0.3 to 2. Since the calculated mass transfer coefficients mere of the same order of magnitude as the calculated reaction rate constants, i t is expected that for the small particles a t low Reynolds numbers mass transfer does play a role in controlling the rate of reaction. At the highest flow rate, the Reynolds number approaches the lowest value used with the largest particles; the region where it had previously been shown that mass transfer was not a large factor. Therefore, the limiting coilversion ratio for the two particle sizes appears to be about 4. Thus, the effect of pore diffusion is quit,e appreciable even though it is not equivalent to that expected in the region of st,rong pore diffusion. Since the catalyst part'icle size used in the initial experiments was t,he main fraction of t,he commercial catalyst, and also since the conversion variat'ion with particle size is not controlling, the postulated rate equation should be a good model for t,he reaction in an industrial gas polymerization reactor. Studies (Ipatieff, 1935; Langlois and Walkey, 1951) have been made of the reaction rates for the dimerization of individual olefins. The reaction rate increases in the order: propene, 1-butene, 2-butene, isobutylene. It was also shown that isobutylene has an effect on the rate of polymerization of 200/ 180-
1.60-
IIC
V/F
ICC CATALYSTIMOLE OLEFINIHRI
Figure 3. Linearity tests for different rate equations 550
Ind. Eng. Chem. Process Des. Develop., Vol. 10, No. 4, 1971
VIF
I C C . CATALYSTIMOLE O L E F I N I H R I
Figure 4. Fit of Equation 2 to experimental data
Io08-
g06D
8
:04-
z G02-
g "0
0-
-02-
-04 460-F 1 0 1 ,
420'F
3gO"F
340'F
MO'F
I
propene a n d butene-that is, interact,ion effects are appreciable. T h e individual olefin conversions found in this study as a funct'ion of the total olefin conversion are shown in Figure 6. Considering only the main components of the feed-Le., propene a n d the various butene isomers-the order of increasing reactivity would be: T-2-butene, C-%butene, propene, isobutylene, aiid 1-butene. T h e lack of agreement of this order with that of the previously mentioned workers is probably owing to the isomerization reactions which are occurring. This would also explain the negative conversions shown in Figure 6. Discussion
A number of assumptions were made in the present study a n d will now be examined. Constant catalyst-bed temperatures have been assumed. However, small temperature differences were found betweeii the beds. A t 300'F, a difference of 1-3°F was found while a t 460'F a difference of from 10-13°F occurred. Because of the low activation energy for the reaction, the resultant niaxiniuni errors in the reaction rate would be only 2-37, a t 300'F and about 107, a t 460'F. The overall errors in the calculation of the conversion would be less than this since each value reported is the average for several runs. Constant catalyst activity has been assumed during a daily series of runs (8- to 12-hr period). Since fresh catalyst was used each day, this should be a fairly good assumption. a check, repeat runs were made a t the start a n d finish of the series of runs. Good agreement was always found. Langlois and Walkey (1951) have shown that the phosphoric acid concentration of the catalyst is important' in determining its activity. In industrial reactors water is added to the feed before it enters t'he reactor to maintaiii the catalyst iri t,he same state of hydration. In this work no water was added. However, as pointed out' previously, no change in catalyst activity was found betweeii the st,art.a n d the fiiiish of a day's run. Calculations of the rate constants were made, with plug flow through the catalyst beds assumed. Residence time distribution studies were not made on t8hereactor, but., because of the short length of each bed aiid the careful packing of the catalyst between the screen supports, this appears to be t81ie best assumption. Conclusion
The experimental kinetic data obtained for t'he polymerization of a commercial olefin mixture were correlated well by ail empirical rate equation, whereas the rat,e equation postulated by Langlois a n d Kalkey (1951) for poljmierization on a liquid phosphoric acid catalyst did not represent the data well. Ilow-
ever, the activation energy reported by Laiiglois and Kalkey for the reaction on a liquid phosphoric acid catalyst agreed closely with the activation energy found in this work with a commercial solid phosphoric acid cat'alyst. Since the test, coiiditions were quite different (olefinic niist'ures ill t'liis work compared to pure olefins in Laiiglois and Ralkey's work) it would have been fortuitous if the rat'e equations h a d agreed more closely. The similarity in the values of the activation energies in the two cases would indicate that t,he reactioiis which occur during the polymerizatmioliof olefiiis are very similar with the two types of catalyst. It was found that pore diffusion affect'ed the overall conversion with the commercial catalyst. However, tlie kiiietic model proposed in this work, which fits well the conr-ersion data obtained wit'h the size fraction most, represeiit,ative of t,llp industrial cat.alyst, should hold well for modeling a full-scalc plant even if t,liere is a small amount, of cat:ilyst .~hnt,teriiig during the dumping and leveling of tlie bed.. Ac knowledgrnent
T h e management of Shellburn Refinery of Shell Oil e o . of Canada have been very helpful iii supglyiiig information, as well as catalyst and feed stock used i l l these experiments. Nomenclature
Co = initial olefin concentration, mol o1efiii;'cc feed F = molar flow rate of olefins, mol olefin/hr k = reaction rate const'ant, cc olefin/hr,'cc catalyst r = rate of reaction, mol olefin/hr/cc catalyst R = gas constant, cal./g-mol O K T = temperature, OK V = volume of catalyst, cc z = fractioit of olefins converted iiito product literature Cited Egloff, G., Welnert, P.C., Proceedings of the Third 11 leum Congress, Sec. I V , The Hague, the S e t h e r l a i ~ d ~ p , 201, 19j1. Farkas, A., Farkas, L., I d . Eng. Chem., 34, 6, 716 (1942). Ipatieff, V. S . , U.S. Patent 1,960,631 i l I a y 29, 1934). Ipatieff, V. X., U.S. Patent 2,018,065-6 (Oct. 23, 1935). Ipatieff, V. N., Corson, B. B., Zncl. Eng. Chenz., 27, 1067 (1935). Langlois, G . E., Walkey, J. E., Proceedings of the Third World Petroleum Congress, Sec. IV, The Hague, the Setherlands, p 191. 1951. Levenspiel, O., "Chemical lieaction Engineering," p 456, JViley, New York, N.Y., 1962. Schmerling, L., Ipatieff, V. N., "Advances i n Catalysis aiid Ilelated Subjects,'' Vol. 11, pp 27-54, Acadeniic Press, Kew York,
X.Y., 1950. Shanley, W.B., Egloff, G., Oil Gus J., 116 (May 18, 1939). Iti:ci:rvic~ for review Sovember 12, 1970 ACCI,:PTI.:U hIarch 29, 1971 The authors thank the National Research Council of Cnziadii and the Universitv of British Columbia for their firinncia1 h i i _ D D.o r t of this research.' Ind. Eng. Chem. Process Des. Develop., Vol. 10, No. 4, 1971
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