October 1952
INDUSTRIAL AND ENGINEERING CHEMISTRY
explained on the basis of this mechanism with a constant IC/C*, essentially affected only by temperature and catalyst nature and amount. However, one serious shortcoming to the use of this development is that the equations for the prediction of conversion for an arbitrary schedule of gas rates or for reactors after the first one in a series cannot be integrated, either in a closed form or graphically. Numerical integration can be performed, but it is tedious and does not give any improvement over the results which can be obtained quite easily with the empirical approach. Thus the empirical half-order mechanism has been retained because of its greater utility.
Conclusions These results show that the hydrochlorination of lauryl alcohol with zinc chloride catalyst may be expressed as a nonreversible reaction, half order with respect t o the alcohol. The rate constant of such an interpretation is greatly influenced by temperature, gas rate per unit volume of liquid, and gas purity; effects of these variables are given in the text. They also show that a very effective way of carrying out this reaction is the use of a jet mixer as a recycling device to bring the discharged and unreacted hydrogen chloride back into contact with the lauryl alcohol. Other methods of increasing the fractional utilization of hydrogen chloride are discussed, and the effectiveness of feeding this gas a t a constantly reducing rate was demonstrated experimentally. A more fundamental kinetic interpretation is presented in which the importance of diffusion is incorporated. This shows that the mechanism is probably one of a diffusional equilibrium between hydrogen chloride in the gas and hydrogen chloride dissolved in the liquid, followed by a reaction &st order with respect to this dissolved hydrogen chloride. Such an interpretation leads to a constant, dependent only on temperature and ratalyst nature and amount.
Acknowledgment The first author wishes to acknowledge the financial assistance of the AMERICAN CHEMICALSOCIETYthrough its fellowship program.
2487
Nomenclature
C
= =
f
G k
= = =
kl/2
=
Cz
kS12 =
M = N = NA = P = t
=
z y
=
V = Vl = Vz = W = =
molar concentration, gram moles per liter distribution constant for hydrogen chloride between vapor and liquid phases weight fraction of catalyst in a sample total gas flow rate, gram moles per hour reaction rate constant reaction rate constant obtained by interpreting data as a half-order reaction reaction rate constant obtained by interpreting data as a 2.5-order reaction average molecular weight of alcohol normality of sodium hydroxide solution gram moles of alcohol weight fraction of alcohol in a sample time, hours volume of liquid phase at reaction temperature, Biters titer for a blank in acetylation procedure, ml. titer for a sample in acetylation procedure, ml. weight of sample, grams mole fractional conversion mole fraction of a component in the gas phase
Subscripts
A B
0 1
= component in general or lauryl alcohol = component in general or hydrogen chloride = initial value =
exit value
Superscripts
n
= apparent order of reaction
Literature Cited (1) Chem. Eng. News, 29, No. 28, 2856 (July 9, 1951). (2) Guyer, A,, Bider, A., and Hardmeier, E., Xelv. Chim. acta, 20,
1462-7 (1937). (3) Smith, D. M., and Bryant, W. M. D., J . Am. Chem. floc., 57,61-5 (1935).
RECEIVED for review October 3, 1951. ACCEPTED May 19, 1952. For material supplementary to this article, order Document 3598 from American Dooumentation Institute, 1719 N Street, N. W., Washington 6, D. C., remitting $1.00 for microfilm (images 1 inch high on standard 35 mm. motion picture film) or $1.35 for photocopies (6 X 8 inohes) readable without optical aid. This paper is based on a dissertation presented by Henry A. Kingsley, Jr., to the faculty of the Yale School of Engineering in partial fulfillment of the requirements for the degree of Doctor of Engineering.
laboratory Continuous Distillation Column For Concentration of Aqueous Solutions of Volatile Flavors
Engrnyring Process development
K. P. DlMlCK AND MARION J. SIMONE Western Regional Research Laboratory, Albany, Calif.
I
N THE study of the chemistry of fresh-fruit volatile flavors, a
fractionating column was required for concentrating the large amounts of very dilute aqueous solutions of the flavors which were obtained by flash evaporation (3) of fruit purees. For this purpose an all-glass laboratory fractionating column of high plate efficiency and high throughput was desired. Furthermore, the material should not be subjected to prolonged heating, which might accelerate decomposition or alteration of the fruit flavors. A modified, continuous-feed, Oldershaw bubble plate column met these requirements. This type of column as described by
Oldershaw (6) and improved by Collins and Lantz ( 2 ) consists of a series of perforated glass plates sealed into a tube. Each plate is equipped with a weir, t o maintain the proper liquid level on the plate, and a reflux return tube directing the liquid to the next lower plate. The conventional column, which was designed for nonaqueous distillations, is not suitable for the distillation of aqueous solutions ( 2 ) because of the high surface tension of water, which causes flooding and abnormal pressure drop across the column. This limitation was overcome by increasing the distance between plates and diameter of the plate perforations.
2488 The efficiency and operational characteristics of the column in concentrating volatile organic material fromverydilute water solutions were determined by use of a modelsystem consisting of a 0.005% solution of phenol as the test mixture, and by determining the recovery of the volatile materials from a strawberry e s s e n c e solution.
Column Design
INDUSTRIAL AND ENGINEERING CHEMISTRY Solenoid
overflom tube served two other purposes: as a water manometer indicating the pressure drop across the column and as an inlet tube for filling the kettle. The kettle was maintained about one-half full of water by adjusting the height of the overflow tube, and was heated with a 200- and a 500-watt stainless steel, 0.25-inch calrod-type immersion heater. The boil-up rate could be varied from 200 to 1200 ml. per hour by controlling one of the heaters with a variable transformer. A 5-liter round-bottomed flask with a Nichrome immersion heater has also been used as a kettle. Constant feed to the column was provided by a small stainless steel gear pump coupled to a variable-speed drive which permitted a feed rate of 0 to 2 liters per hour. Since the column operates a t about 100' C. (212" F.), the pumped liquid was heated to the same temperature by atmospheric steam in a glass preheater 6 inches long just before entering the column.
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Reflux Condenser
Vol. 44, No. 10
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-
Automatic Vapor Dividing Stillhead
L
1591ate Column_
k d u d Condenser
L
r
-
A 40-plate column 3 0 t h Plate was built according to the approximate s p e c i f i c a t i o n s described by Collins and Lantz ( 2 ) (Figure 1). The column, of 28mm. i n t e r n a l d i 2 0 t h Plateameter, was built in two equal sectionseach section having 25Flote C o t 3 an inner and an outer 29/42 standard glass SOut teL SIn Lm joint. The sections, Filling F Y including the ground joints, were vacuum 10th jacketed andsilvered. One section con3J tained 25 perforated lates and t v o inlet eed points a t the 10th and the 20th plate from the Feed In bottom. The other section contained 15 plates with a feed point a t the 5th plate from the b o t t o m . T h e c o l u m n was built in two sections -Sight Glass for the following reasons.< A s i n g l e c o l u m n of s u c h length as to accomm o d a t e 40 plates would have inherent 8 t r u c t u r a1 we a knesses. The second reason was that the two sections could Figure I. Continuous-Feed Distillation Column a n d Accessories be i n t e r c h a n g e d , thereby p r o v i d i n g greater variation in the feed positions with respect to the kettle. The feed tubulations projected approximately 1 inch from the column and terminated in 12/5 inner ball joints. The plates in the column had the following specifications. The distance between the plates was 1.375 inches, hole size in the plates was 0.015 inch in diameter, and the number of perforations per plate was 50. Two heads were used: a partial-condensation type through which constant-temperature water was circulated and the positive-acting vapor-dividing head described by Collins and Lantz (g). The latter was found more satisfactory because it permitted a more accurate control of the rate of take-off. The rate of takeoff was controlled by an electronic repeat cycle timer. The kettle, having a capacity of 5 liters, was made from a 6inch diameter stainless steel cylinder with hemispherical ends and with a 29/42 stainless-steel inner joint welded to the top. The kettle was equipped with a drain port, a sight glass, and a glass overflow tube the height of which could be adjusted. This
T;I
Operation of the Column The kettle is first filled to one-half full with water through the overflow tube, and the voltage to the heater adjusted to give the desired boil-up rate. As the water begins to boil the pressure in the kettle increases, offing to the resistance the plates offer to the vapors ascending the column. This is indicated by a rapid rise of the level of the water in the overflow tube. After a constant pressure in the column has been established, as indicated by a constant level of water in the overflow tube (about 10 minutes is usually required after boiling starts), the height of the overflow tube is adjusted until the water in the tube just begins to flow over. This will be about 32 inches above the liquid level in the kettle. The solution to be distilled is then pumped into the column. The distillate is collected at the desired rate by adjusting the timer, which controls the valve in the vapor dividing head. A setting of 0.5 second o en and 12 seconds closed mill result in a take-off rate of about 5910 A boil-up much greater than 1000 ml. per hour will cause flooding in the column. After the feed solution has been exhausted, water is used as feed for a few minutes to flush the feed lines. The distillation is, however, continued (without feed) for a much longer time to recover the solutes held up in the column.
PreheT
P ;
-
D
Performance of Column Inasmuch as the still was constructed for the concentration of extremely dilute solutions of unknown organic flavor substances, it was considered more significant to evaluate the performance of the column by means of direct-recovery measurements from a model system rather than to evaluate the plate efficiency of the column. Such a system should contain a volatile solute in very low concentrations, which is difficult to separate from water and which could be easily determined quantitatively in low concentrations. A system which has proved very satisfactory consists of a dilute solution (0.005%) of phenol. This concentration was chosen because it represents approximately the concentration of the volatile flavor components found in fruit. Phenol and water form an azeotrope which boils a t 99.6"C. and has a composition of about 9% phenol a t atmospheric pressure (6). Because of the slight difference between the boiling points of the water and of the azeotrope, an efficient column is required to effect appreciable concentration of the phenol. Phenol can be determined quantitatively by a modification of the method of Folin and Ciocalteau (3). By the use of this test solution the operational characteristics of the column were investigated to determine: (1)the time required to reach equilibrium, (2) the per cent of phenol which can be recovered from the feed, and (3) the time required to remove most of the phenol from the column after the phenol feed has been interrupted and the water feed has been started. During a typical run the column was operated in a continuous manner, a t near the flood boil-up rate (950 ml. per hour) a t a feed rate of 940 ml. per hour, and a t a take-off rate of approximately 50 ml. per hour, for a period of 3 hours. The distillates were collected for each successive 15-minute distillation period and analyzed for phenol. The per cent phenol recovered in the distillate for each 15-minute period (calculated on the basis of the amount fed to the column during the 15 minutes) was plotted against time (Figure 2). For convenience in expressing the results, the recovery of phenol,
October 1952
INDUSTRIAL A N D ENGINEERING CHEMISTRY
after phenol feed was discontinued, was calculated on the same basis as before. About 60 minutes are required to establish an equilibrium concentration gradient in the column as indicated by a constant phenol recovery of 83%-the remaining 17% being lost to the kettle. After feed is discontinued, about 45 minutes are required to reduce the product concentration to a low value (about 7% recovery). A limited investigation was also made on the following variables: take-off rate, feed position (feed plate location), boil-up rate, and feed rate. I n all cases the efficiency of the column was based on the recovery of phenol from the 0.005% phenol test solution after equilibrium conditions had been attained in ‘the column. The values are shown in Table I.
Table 1. Operating Characteristics of Modified Continuous-Feed, 4O-Plate, Oldershaw Column Feed TakeFeed Phenol offb, Plat:, ReoovRate Rate, Ml./H&r Ml./Hour % No. eredd, % 87 940 7.2 30 950 30 84 950 2 vs 940 5.0 3: 0 940 30 82 vs 950 3 7.2 20 87 vs 950 940 4 83 950 5.0 940 20 5 vs 20 83 vs 950 3.0 940 6 75 vs 10 950 5.2 940 7 10 73 3.4 940 8 vs 950 20 vs 475 87 7.0 470 9 6.0 20 84 D 985 980 10 87 D 6.5 20 985 485 11 4.5 20 D 1500 65 12 985 10 71 6.2 940 vs 950 136 5 VS indicates vapor-splitting head: D indicates dephlegmator head. b Based u on feed rate. C Countec?from bottom. d Measured after equilibrium had been established (see legend Figure 2). 0 20-Plate column.
Expt. NO. 1
Head Type” VS
recovery of phenol a t a boil-up of 950 ml. per hour (which is near maximum), a feed rate of about the same, and a t a take-off rate of as low as 3% of the feed rate, and with the feed position a t the 20th or 30th plate. With the optimum operational conditions established for phenol, the performance of the column in concentrating a strawberry essence was tested. This was done by determining the amount of organic carbon in a flavor solution before and after concentration. Carbon was determined by the wet combustion procedure described by Christensen et al. (1). An approximately 30-fold essence solution (4400 grams containing 6.65 grams of carbon) was distilled at a boil-up rate of 950 ml. per hour, a feed rate of 950 ml. per hour, and a take-off rate of 5% of the feed rate, thus concentrating the feed 20 times. The final distillate (270 ml.) represents an approximately 500fold essence concentrate and includes the distillate collected during the time required for reaching equilibrium and for stripping the column at the end of the run. This concentrate contained 6.48 grams of carbon, representing 97.4% of the carbon in the feed material. Redistillation of the spent liquor (still bottoms) resulted in an additional recovery of 1.3% of the carbon, leaving 1.3% of the carbon unaccounted for. The limit of accuracy in the method for determining carbon is not sufficient to make this discrepancy meaningful. The recovery as measured by carbon does not necessarily represent a uniform recovery for the different organic components of strawberry essence; however, the strawberry aroma in the still bottoms prior to the second distillation (in the experiment above) was extremely weak, suggesting that most of the important flavor components had been removed. 100,
Reducing the rate of take-off from 7.2 to 3.0% of the feed rate does not greatly decrease the recovery of phenol as shown in Experiments 1 to 6. Comparing Experiments 1 with 4,2 with 5, and 3 with 6, indicates that equal efficiency in recovery of this test solution is achieved with feed positions a t either the 20th or the 30th plate. Feed position at the 10th plate, however, decreases the recovery of phenol, as is shown by comparing Experiments 7 and 8 with 2 and 3 or with 5 and 6. A lower operating rate (smaller boil-up and feed rate) did not improve the efficiency of the column as shown in Experiment 9, in which the same phenol recovery was obtained as in Experiments 1 and 4. Experiments 10, 11, and 12 were performed with a dephlegmator head. An 84% recovery of phenol was obtained in Experiment 10, which compares with recoveries in Experiments 4 to 6, thus indicating that the two types of heads have about equal efficiencies. By reducing the feed rate to approximately one half the boil-up rate, only a slightly greater recovery of phenol was obtained as shown by comparing Experiments 11with 10, in which 87 and*84% recoveries were made, respectively. Increasing the feed rate, however, to approximately 50% greater than the boil-up rate, as shown in Experiment 12, caused a marked decrease in efficiency, resulting in only a 65% recovery of phenol. To gain some idea of the relative efficiency of a smaller still, a 20-plate continuous column of identical construction was tested with the feed position a t the center of the column. The recovery of phenol, as shown in Experiment 13, was 71%, compared with 83 to 87% obtained with the 40-plate still, also fed at the center of the column (Experiments 4 and 5). However, as shown in Experiments 7 and 8, the 40-plate column, with feed position at the 10th plate, gave a recovery of only 73 to 75%, which is comparable to the results obtained with the 20-plate column, This indicates that the stripping section (portion below the feed plate) was the factor affecting recovery of the phenol. Although the recovery of phenol is less, the 20-plate column may prove adequate for many applications. It was concluded that the 40-plate column operated with good
2489
I
I
1
. 1
/
I
1
I
I
1
\
83 %
1
Phenol F e e d Discontinued
1 a 60
OO
90
Distillation Time,
Figure
120
150
180
30
Minuter
2. Recovery of Phenol with the rlO-Plate, Continuous-Feed Oldershaw Column
Boil-up rate, 950 m 1 . r hour. Feed rate 950 ml. Der hour. Take-oft rate, 5 R of lee rate. Feed conconhion, 0.005 $4 phenol
It was previously noted that a batch-type still imparted considerable off-flavor to the product in concentrating an aqueous strawberry essence obtained from strawberry puree by vacuum flash evaporation. To test the Oldershaw continuous column with regard to this off-flavor development, the aqueous essence, approximately twofold, from eight varieties of strawberries was concentrated at a boil-up rate of 950 ml. per hour, feed rate of 950 ml. per hour at the 20th plate, and a take-off rate of about 5% of the feed. When the concentrate was reconstituted and compared by the triangle method of organoleptic appraisal ( 4 ) with the original dilute aqueous essence, no significant difference in flavor was found for any of the eight samples. I n another experiment, a 100-fold Gravenstein apple essence was concentrated with this still to about 1500-fold in a single distillation without producing any detectable flavor change. There
2490
Vol. 44, No. 10
INDUSTRIAL AND ENGINEERING CHEMISTRY
was essentially no flavor loss to the kettle as determined by an organoleptic appraisal of the kettle residues.
solution shows the still to be well suited to the requirements of essence concentration.
literature Cited
Conclusion Although it has been reported that the Oldershaw column in its original form ia unsuitable for distillation of aqueous solutions because of the high surface tension of water (W), the modifications used in the present experiments proved satisfactory for such distillations. The evaluation of the operating characteristics of the 40-plate continuous column through the use of the phenol test solution and the recovery measurements of a strawberry flavor
(1) Christensen, B. E., Wong, R., a n d Facer, J. F., ISD.ENG.CHEM., ANAL.
ED.,12, 364 (1940).
(2) Collins, I”. C . , and Lantz, V., Ibid., 18, 673 (1946). (3) Dimick, K., and Makower, B., Food Technol., 5, 517 (1951). (4) Helm, E., and Trolle, B., Wallsrstein Lab. Communs., 9, 181
(1946).
(5) Horsley, L. H., Anal. Chem., 19, 508 (1947). (6) Oldershaw, C. F., IND.ENG.CHEX, ANAL. ED.,13, 265 (1941). R ~ C E I V for E Dreview February 11, 1952.
ACCBPTED May 23, 1952.
Flame Velocities of liquid Hydrocarbons
development
R. E. ALBRIGHT, D. P. HEATH,
AND
R. H. THENA
Socony-Vacuum Laborafories, Paulsboro, N. J.
HE evaluation of fuels for gas turbine engines requires an experimental or laboratory test method Yhich can be made to correlate with performance in full scale equipment. Over a period of several years many methods have been devised for the evaluation of conventional aviation, motor, and Diesel fuels. Presumably, a similar evolution of methods \vi11 grow out of gas turbine fuel studies. Present knowledge indicates that the velocity of flame propagation of a fuel is an important rharacteristic of gas turbine fuels. Correlations between flame stability (blowout limits) and relative flame velocities have been obtained. In addition to the basic fuel flame propagation rate, the minimum ignition energy
T
requirements, flame temperature characteristics, and the quenching effect of relatively cold solid surfaces upon laminar gas flames have been investigated. On the basis of this work it appears that some combination of these fuel characteristics may provide a more complete expression of fuel combustion quality. The present discussion is limited to the study of the flame velocities of normally liquid hydrocarbons. The data presented are the results of an investigation initiated at the SoconyVacuum Laboratories several years ago as part of the general studies on the combustion characteristics of aircraft gas turbine fuels, Only a limited amount of information was available in the literature on the flame velocities of normally liquid hydro-
MIXER a VAPOR I 2 0R
AIR
I
I
TEAM I N L E T
-
. I C E TOWER CRITICAL-FLOW ORIFICES
a 3
SUPPLY
,-+clh
L
AIR
FLASH B A C K
FUE
I
RESERVOIR HEATER AIR SURGE T A N K
STEAM
OUTLET
____ Figure 1.
Diagram of Flame Speed Apparaiur
