ehvdration of over A h J
CHARLES A. WALKER Yale University, Akw Haven, Conn.
This mechanism was first suggested by Ipatiev ( 7 ) and was supported by the studies of Pease and Yung (9),Senderens ( l a ) ,arid Alvarado (5'). Sdliins and Perkins ( 1 ) showed, however, that thr yield of butylene from dibutyl ether is considerably less than thc yield of butylene from butyl alcohol when the two are comparcd in floiv systems a t like feed rate, temperature, pressure, and catalyst volume. This result suggests that some direct dehydration of the alcohol t,o olefin must occur and t'hat a t,hird rcact,ion must bo considered:
T h e results obtained on dehydrating 1-dodecanol at varying feed rates 0%er alumina at pressures of 40,160, and 320 m m . of mercury absolute and temperatures of 302', 318 ', 339 O , and 358" C. are reported. Higher temperatures and low feed rates lead to high yields of olefin with conversions as high as 9 7 q ~ . Maximum conversion to ether is 40%. The mechanism of alcohol dehydration i s discussed.
T
HE dehydration of normal aliphatic primary alcohols over
IZCH,CH?OH +BCH=CH,
alumina in constant-pressure flomsyst,emsresultiiirithe formation of the corresponding olefin or ether or a mixturr of the t,wo. By suitable regulation of the temperature and flow rate, it is possible to obtain the olefin as the main product, and several investigators have reported studies made under such conditions (3, 4,6, 8). I n general, high temperatures and low flow rates are n e c w sary to obt.ain olefin as the primary product,. At lower temperatures and higher flow rates mixtures of the olefin and ether are obtained. Several investigations in 1%-hichboth products we^^+ obtained have been reported ( 1 , 2, 7 , 9,1%, 14). I t has not been found possible to produce pure rther without olefin formation except by the use of higher pressures ( 1 1 ) . The reactions involved when an alcohol is d e h y d r a t d under conditions which lead t o formation of both olefin and ether arc of scientific interest because of the mechanism. It has been considered that the ether is an intermediate product in olefin formation and that the reaction scheme is:
2RCHrCHsOH
-++
+ + R?O
ItCHsCH~OCH~CIlplil H a 0
RCHyCHnOCH2CH2R ---+2RCH=CHz
(1 )
(2)
(3)
Thus it is conceivable that the reaction mechanism may involve simultaneous reactions 1 and 3, consecutive reactions 1 and 2, or H combination of the t.hree reactions. It. appears prohable that tht: conditions under which the studies of Pease and Yung (9), Sendereris (12), arid Alvarado ( 2 ) were made may have been such that most of the ol&n was formed by ether dehydration. Yone of the data available on dehydration of alcohols to olefill arid ether are satisfactory for mechanism st,udies because of toinperature gradients in the catalyst bed. Most of the studies so far reported were based on temperature a t some single point in t h e reactor. Swann and Kearby ( 1 4 ) showcd that an appreciabk temporature gradient is noted in the case of the lower alcohols even when t,he catalyst is contained in B thin annular space. Furthermore, studies of olefin and rther formatmionhave not, been reported for the higher normal alipha.tic primary alcohols: although several studies on the dehydration of such alcohols to olefins ttppear (3,4,8). The purpose of this study was to obtain data concerning yields of olefin and ether obtained by the vapor-phase dehydration of w
TO FAANOSTAT
STEEL TANK
Figure 1.
+ HzO
Glass Equipment for Dodecanol Dehydration
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INQUSTRIAL AND ENGINEERING CHEMISTRY
November 1949
’0.125” BORE
0.500”BORE
f
0.025” BORE
L
2641
sion heater (1 kilowatt). The temperature of the salt bath is controlled manually by adjustment of a voltage regulator on the immersion heater. Differential manometer M , for observing pressure drop across the catalyst bed, is placed as close a s possible to the reactor system in order to minimize condensation of vapor in the manometer lead. This manometer has a check valve in each arm to prevent blowing mercury into the system during evacuation or filling the system with nitrogen. Primary condenser G, receiving flask H , ice traps J , and surge tank K require no description other than that which is obvious from Figure 1. Nitrogen is admitted to the previously evacuated system as shown t o set the pressure at any desired value. The pressure is regulated by a mercury manostat (not shown) which actuates the solenoid valve. I n using this equipment it was found necessary t o degas the dodecanol by subjecting it t o a vacuum (about l mm. of mercury) for 30 minutes with occasional agitation. Otherwise a mixture ot liquid and gas appeared in glass tube D and led t o a n intermittent feed to the reactor.
ED 0,025”sQUP THREAD
Figure 2.
Adjustable-Length Capillary Thread
higher alcohol (1-dodecanol) over alumina in a constant-pressure flow system. The variables of greatest interest were feed rate, temperature, and pressure. The data were obtained under such conditions t h a t the temperature in the reactor varied no more than * 3 C. and should be suitable for mechanism studies. O
APPARATUS
Figure 1 shows the glass equipment developed. The dodecanol feed is contained in reservoir A , which is used for feeding except during flow-rate measurements when the feed is entirely from buret B. Since no proportioning pump was available when this work was started, it was necessary to regulate the feed rate by capillary thread C, shown in greater detail in Figure 2. The feed regulator consists of a 0.025-inch-square thread on a 0.500-inch-diameter steel rod. The rod fits in a smooth tube with a bore of about 0.500 inch. Thus the feed liquid flows through a capillary thread, and the feed rate is adjusted by regulating the length of this thread. By-pass E is provided t o catch the first liquid coming through the feed regulator, since this liquid usually contains appreciable quantities of dissolved air. Vaporizer F consists of a glass U-tube, 0.8-inch inside diameter arid about 12 inches long. The vaporizer is packed with l/d-inch lengths of 6-mm. glass tubing. The vapors from the first U-tube flow down over the catalyst which is contained in one a r m of a second U-tube. The inside diameter of the catalyst chamber is 0.8 inch. A Pyrex tube of 7-mm. outside diameter extends in this chamber t o the catalyst support. A copper-constantan thermocouple in this tube is used t o measure the temperature at any horizontal level in the reactor. The entire assembly of vaporizersuperheater and reactor is heated by immersion in a bath of molten heat-transfer salt. This salt bath is heated by a n immer-
‘0
0.1 a2 0.3 0.4 FEED RATE, G.-MOLES/ HR.
0.5
Figure 3. Effect of Feed Rate on Percentage Conversion to Olefin
The feed regulator functioned satisfactorily in the early runs, but in some of the later runs i t was found necessary to provide a seal of liquid dodecanol by immersing the extended portion of the rod and the lower end of the tube in dodecanol. A small amount of dodecanol leaked into the system from this seal, so t h a t actual feed rates after this change was made are somewhat higher than the feed rates determined from the buret readings. I n these cases the product rate was used as the feed rate. This feed regulator has since been used in combination with a constant-head device for a system operating a t atmospheric pressure and found to be very satisfactory. The dodecanol was supplied by the Connecticut Hard Rubber Company [lot 18, run 129 (4)A]. The dodecanol content as determined by acetylation was 100.9%. The melting point was 23.4” C. The catalyst used was grade F-1 alumina, 8-14 mesh (Aluminum Ore Company). The weight of catalyst, in all cases was 50 grams.
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I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
300 Figure 4. Effect of Feed Rate o n P e r c e n t a g e C o n v e r s i o n to E t h e r
PROCEDURES
Bcfore any rutensive experiments were planned, it ivni I ~ P W S sary to make preliminary runs to determine the best operating procedure and the effect of ccv%ain variabks on catalyst activity. The first procedure tested involved using the equipment o n l y 8 hours each day. It was planned to provide an activation period a t the start of each day's run by raising the s d t bath into place after evacuating the system to 2 mm. of mercury and starting the feed after an interval of one hour. This procediirc led to rapid coking of the dodecanol adsorbed on the catalyst. Hence the activation period was eliminated and the feed star ted irnmrdiatelv after the salt bath was raised into place each day. Opwation by this method was satisfactory. The catalyst artivity was found to become constant after the first 5 hours of operation n i t h a new catalyst. Furthermoie, the catalyst activity was constant for the 5 days of the test. Intermittent opeiatioii ITas abandoned in favor of continuous operation because of thc time saving which resulted. The first contiiiuous run was made in thiee periods (18 hours at 319' C., 9 hours a t 339", 9 hours a t 319") a t constant pwssure and flow rate. The catalyst activity again was found to become constant within 5 hours after the run was started. The results obtained a t 339" C. were constant for the duration of this portion of the run; this was also true of the third portion of the run. Thus it appeared safe to allow about 5 hourb for the catrlyst activity to fall from its high initial value to a constant value, and a 4-hour period a t each condition of t e m p r d t u r e and feed rate t o ensure that samples were representat ~ v of e constant catalyst activity. The temperature yradi+rit d o n g the length of the reactor was studied in thrse prrlirninary runs. Since the difference between top and bottom temperatures was never greater than 0.5' C., i t was decided to measure the temperature at the center of the reactor at LL distance of 1 inch from the top and to record only this temperature in subsequent runs. This seemed justifiable in view of the fact t h a t the temperature drop from the salt bath to the center of the reactor was somewhat larger (about 2 " C. at the low flow rates). I n subsequent runs the temperatures of the salt bath and reactor were recordrd a t 15-minute intervals. The t r m p r a -
310 320 330 340 TEMPERATURE, OC.
Vol. 41, No. 11
350
360
Figure 5 . Effect of Temperature on Percentage C o n v e r s i o n to Olefin
tures recorded on the plots of data represent the arithmetic averago of the salt,bath and reactor temperatures. Since the salt bath teniperature was controlled manually, it could not be kept closer than 1O C. to any desired value over a pcriod of hours. The tcmperature gradient across the reactor increased to about 4 " C . at high flow rates. Thus the temperatures recorded on the plots are believed to be within 3" C. of the temperature a t any point in the reactor, and in most C ~ S P Sthey a,re within 2 C. of the temperature a t any point in the rexct,or. The preliminary runs also off ered information relating t o the material bslance sincc the feed rate ri.gulat,or functioned satisfactorily in these runs. I t m m found that the weighed product was 1 to 3% less than the feed rate indicated by determining the time required for 10 ml. of dodecanol to drain from the buret. The cause of this discrepancy was not ascerbained. I t may be due to errors in measuring the feed rate or to some products passing through the ice traps. An attempt was made t o use dry ice lor cooling these traps, but rapid stoppage due t o snow forination complicated this even when the traps were redesigned in an attempt to allow for snow format>ion. I n subsequent runs the fnod rate recorded may be either that measured by the buret (rvht:n thc feed rate regulator n-as operating satisfactorily) or the product rate. The pressure during the preliminary runs was 20 mm. of marcury. Low pressure was used because of the very high boiliiig point of didodecyl ether. However, it was decided t o use 40 mm. of mercury as the pressure in the next run t o increase 1,he c s E ciency of the primary condenser in regard to the water in tlic product stream. This resulted in the expected change, wit!i l i t h material being condensed in the ice t,raps. The results of the preliminary runs were used to schedule a 120hour run (designated as run C) a t 40 mm. of mercury with 50 grams of catalyst, It was desired to obtain yicld data a t four feed rates at each of four temperatures. T o provide additional data on catalyst activity as well as yield data, it appeared advisable to include the following periods in t,his run: (1) 3-hour activation period (302 O C., 2 mm. of mercury), (2) 22-hour period a t 302' C. and constant Ron- rate, (3) four feed rates at 302" C., (4) four
INDUSTRIAL AND ENGINEERING CHEMISTRY
November 1949
feed rates a t 318 C., (5) four feed rates a t 339" C., (6) four feed rates a t 358" C., (7) two feed rates a t 339" C., (8) two feed rates a t 318" C., and (9) two feed rates at 302" C. Each condition of temperature and feed rate was maintained for 4 hours. The product was collected a t the end of each hour. The products from the second and fourth hours a t each condition were analyzed for olefin content. Since these were consistent in all cases, only the product from the fourth hour at each condition was analyzed for alcohol content. A continuous run of about 48 hours (designated as run D) was undertaken t o determine the effect of pressure on the relative yields of dodecene and didodecyl ether in the vapor-phase dehydration of dodecanol over alumina. The entire run was carried out a t approximately 340' C. The operating procedure followed that of run C. It was intended to study yields at pressures of 40, 160,320,480 mm. of mercury. Satisfactory results were obtained at the three lower pressures, but a t 480 mm. didodecyl ether condensed in the reactor. The organic layer was analyzed by determining the olefin content by bromination and the alcohol content by acetylation with acetyl chloride. The method of Rowe, Furnas, and Bliss (IO)was found satisfactory for the iodine number determination if the time of exposure t o the brominating agent was controlled a t one hour. The dodecanol was found t o contain some unsaturation, and a value of 3 for the iodine number was used in calculating results. The iodine number determination was checked on known mixtures of dodecene and dodecanol, and found t o be accurate within 2 units. Since the iodine number of dodecene is 151, the error in olefin content calculated from the iodine number is 1.3%. The method of Smith and Bryant (IS) for determination of alcoholic hydroxyl was found satisfactory. The estimated error in dodecanol content of the samples is 2%. Since the ether content was calculated by difference, it may be in error by as much as about 3%.
2643
O
*
RESULTS
The results of run C are plotted in Figures 3 to 6. That the catalyst activity was constant during run C is clearly indicated in Figures 3 and 4,where the open circles represent data obtained in the third to sixth periods of the run (temperature increasing) and the black circles represent data obtained during the last three periods (temperature decreasing). The data agree within the limits of accuracy of the analytical methods. The catalyst activity in runs C and D may be compared also, since a portion of run D was made at 40 mm. of mercury. Thus, at 339 C. and a feed rate of 0.2 gram-mole per hour, the plots indicate yields of 6101, olefin and 23% ether in run C, as compared with 58% olefin and 25% ether in run D. It appears that the catalyst used is one which rapidly reaches a state of constant activity and maintains this activity for relatively long periods. When a single reaction is carried out by passing a vapor continuously over a solid catalyst, the usual effect of increasing the flow rate is t o decrease the percentage conversion. This is to be expected, since a n increase in feed ratebis equivalent to a decrease in contact time. The effect of feed rate on conversion of dodecano1 is shown in Figures 3 and 4. The effect of feed rate on the percentage conversion of dodecanol t o dodecene is that expected, a decrease in percentage yield with a n increase in feed rate. Similarly, the effect at 302' and 318' C. of feed rate on percentage conversion of dodecanol to didodecyl ether is that expected. The percentage yields of ether at 339" and 358" C., however, increase markedly as the feed rate is increased. An increase in ether yield with a shorter contact time suggests strongly that the ether is dehydrated at a n appreciable rat,e at, these temperatures. Thus the ether yield would go through a maximum somewhere in the reactor, and then the rate of ether dehydration would become greater than the rate of ether formation, resulting in decreasing ether yields with longer contact times. O
300
310
320
330
340 350
360
TEMPERATURE,°C.
Figure 6. Effect of Temperature on Percentage Conversion to Ether
Cross plots showing the effect of temperature on the percentage conversion of dodecanol at constant feed rate may be prepared from Figures 3 and 4. Since most chemical reactions increase in rate as the temperature is increased, it would be expected t h a t the yields would increase as the temperature is increased. That this is the case with regard to dodecene is obvious from Figure 5 . However, the ether yield a t constant feed rate shows a markedly different behavior, increasing at first, reaching a maximum, and then decreasing. There are two possible explanations for the occurrence of these maxima. It is probable that the equilibrium yield of ether may decrease as a result of a n increase in temperature. Unfortunately, data are not available to permit calculation of the equilibrium yields of didodecyl ether. It is also possible that the maxima are the result of relatively rapid ether dehydration a t the higher temperatures. If the temperature coefficient of ether dehydration is much greater than that of ether formation, a maximum yield of ether would be expected. Again, it is not possible to decide whether this is the factor involved unless the complete mechanism of alcohol dehydration can be derived. An investigation of ether dehydration over alumina is indicated, and such a study is being started. A few data on the dehydration of didecyl ether in the equipment used here indicate a very low rate at 320' C. and a rather rapid rate of dehydration a t 365 C. The effect of total pressure on the percentage yields of olefin and ether in dodecanol dehydration was studied in a single run at, 340" C. The results indicated that the yields of ether and olefin a t given feed rate and temperature were substantially independent of pressure in the range 40 t o 320 mm. of mercury. O
MECHANISM OF DODECANOL DEHYDRATION
Dohse (6) investigated the dehydration of several lower alcohols over alumina under conditions such t h a t no ether was obtained in the product. Use of a differential reactor and continuous recycling of the products of dehydration led to data which conformed to EL first-order mechanism as regards the time-dependence of conversion. The observation that the half-life was proportional to pressure led Dohse to suspect a zero-order reaction. Accordingly, a U-tube filled with barium oxide was placed in the recycle line to remove water from the dehydration products. The time dependence of conversion was then found t o conform to
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,
INDUSTRIAL AND ENGINEERING CHEMISTRY
a zero-order mechanism. Dohse concluded that alcohol dehydration over alumina is a zero-order reaction, but t h a t the time dependence of conversion conformed to a first-order mechanism because a product of the reaction (water) is adsorbed on the catalyst surface and renders ineffective a portion of this surface. The range of conversions covered by the data reported here was not great enough to permit a complete kinetic study. It appeared worth while, however, to attempt a n interpretation of the data based on Dohse's conclusions. It was assumed first t h a t only reactions 1 and 3 were involved-Le., t h a t the rate of ether dehydration is negligible. The further assumption t h a t the time dependence of conversion would conform to a first-order mechanism made i t possible to set up the differential rate equations. Substitution of data obtained in this work in the integrated form of the rate equations revealed that the conversion data a t 302 O and 318 O C. are correlated satisfactorily-Le,, the time dependence of conversion conforms to a first-order mechanism. The data at 339" and 358" C. obviously could not be interpreted by these equations, since the ether formed increases with increasing feed rate. The differential rate equations for reactions 1, 2, and 3 were set up and integrated, again on the assumption that the time dependence of conversion would conform to a first-order reaction, but the data were inadequate for a test of these equations. Consideration of the pressure effect indicates, however, a zeroorder reaction. At a constant feed rate and temperature the contact time is almost proportional to pressure. Thus doubling the pressure would approximately double thc contact time. It was observed, however, that conversion was independent of pressure a t constant feed rate and temperature. This is equivalent t o Dohse's observation that the half-life was ncarly proportional to pressure. DISCUSSION
The dehydration of alcohols t o olefins and ethers has been known for some time to involve consecutive and simultaneous reactions. This particular set of reactions is well adapted t o kinetic studies for several reasons. The catalyst, alumina, is readily
Vol. 41, No. 11
available, easily activatcd, and relatively constant in activity There is little by-product formation. It appears that the system may offer some simplification in that simultaneous reactions only are involved at lower temperatures whereas the same simultanoous reactions and a consecutive reaction occur a t the higher temperature. D a t a covering a wider range of conversion in alcohol dehydration, together with data on ether dehydration, should provide for some interesting applications of the theory of heterogeneous catalysis. ACKNOWLEDGMENT
The author is indcbted to R. H. Bliss, who directed the investigation reported here. C. M. Doede, R. J. Humphrey, John Wilkinson, and Victor D'Anzicco of the Connecticut Hard Rubber Company assisted by supplying materials and by providing help in the longer runs. F. Pierce Noble constructed thc glassware and cooperated with the author in developing a satisfactory design. LITERATURE CITED
Adkins and Perkins, J . Am. Chem. Soc., 47, 1163 (1925). Alvarado, Ibid., 50, 790 (1928). Appleby, Dobratz, and Kapranos, Ibid., 66, 1938 (1944). Asinger, Be?., 75B, 1247 (1942). Brown and Reid, J . Phys. Chem., 28, 1077 (1624). Dohse, 2. physik. C h ~ m .B5, , 131 (1929) ; B6, 343 (1930). Ipatiev, Ber., 37, 2968 (1904). Kornarewsky, Uhlick, and Murray, J . A m . Chem. Soc., 67, 557 (1945). Pease and Yung, Ibid., 46, 2397 (1924). Rowe, Furnas, and Bliss, IND.ENG.CHEM.,ANAL.ED.,16, 371 (1944). Semerano, Gazz. chim. ctal., 66, 162 (1936). Senderens, Bull. SOC. chim., 35, 1144 (1924). Smith and Bryant, J.Am. Chpm. Sac., 5 7 , f j l (1935). Swann and Kearby, ISD. ENG.CHEM.,32, 1607 (1940). RECEIVED April 24, 1948. Based on a dissertation presented by Charles A. Walker in June 1948, t o the faculty of the School of Engineering of Yale University, in candidacy for the degree of doctor of engineering.
Emulsion Behavior of Soap-
Stabilized Hevea Latex I
