April 1950
INDUSTRIAL AND ENGINEERING CHEMISTRY
compose to give free sulfur or a vulcanization accelerator. Although it,s compounds should include both magnesium oxide and zinc oxide, Type W also requires acceleration. Many convent,ional neoprene and rubber accelerators can be used. Certain of the rubber accelerators, such as diphenylguanidine or the butyraldehyde aniline condensation products require sulfur. Others, such as 2-mercaptothiazoline and tetramethylthiuram monosulfide, as well as the common neoprene accelerators, such as the di-o-tolylguanidine salt of dicatechol borate, may be used without sulfur. Neoprene Type W vulcanizates differ from those of Type GN by ha,ving a stress-strain curve more nearly approaching that of natural rubber and by having much greater resistance
691
to compression set. The resistance to compression set of Neoprene Type W vulcanizates is greater than that of the best natural rubber stock. LITERATURE CITED
( I ) Bridgwater, E. R., IND. ENC.CHEM.,26, 33 (1934). (2) Bridgwater, E. R.,and Krismann, E. H . , Zbid., 25, 280 (1933). (3) Catton, N. L., Fraser, D. F., and Forman, D. B., du Pont Co., Rubber Chem. Div., Rept. 40-2, 15 (1940). (4) Mayo, L. R . , IND. ENG.CHEM.,42, 696 (1950). (5) Starkweather, H. W., and Walker, H. W., Zbid., 29, 872 (1937). (6) Torrance, M. F., and Fraser, D. l;.,Zbid., 31, 939 (1939). RECEIVED Septemher 26, 19-29.
Stability of Burner Flames with Propane-Hydrogen Mixtures T h e Pennsylvania S t a t e College, S t a t e CoEleqe. I'u.
Blowoff and flash-back data are reported for propane, hydrogen, and mixtures of the two gases. The results obtained i n the region of laminar flow using air-cooled burner tubes may be correlated satisfactorily in terms of velocity gradients at the burner wall. The effect upon blowoff and flash back of increasing concentrations of hydrogen in hydrogen-propane mixtures is shown to be nonlinear for both blowoff and flash back.
T
HE stability and structure of burner flames is a matter of
considerable theoretical interest and technical importance. In recent years significant progress has been made toward an understanding of this subject a5 a result of contributions by Lewis and von Elbe ( 3 )and von Elbe and Mentser ( 2 ) . Using n a t u r d gas, hydrogen, and acetylene, these investigators satisfactorily correlated flame stability in terms of fuel-air ratio and velocity gradient a t the burner wall, independent of burner tube diameter or construction materials. The correlations hold in the region of laminar flow where most of the experimental data were obtained. Bollinger and Williams (I), using propane, have extended the correlation into the region of turbulent flow by the use of appropriate velocity gradient calculations and have shown that for blowoff, the correlation holds satisfactorily u p to a velocity gradient of about 20,000 seconds-'. Because of the interest in the behavior of mixed fuel gases, the present investigation was undertaken to determine whether the velocity gradient concept was applicable to binary mixtures and also to furnish the basis for later studies on multicomponent gases. Mixtures of hydrogen and propane were chosen for the initial study because data were available on these gases separately, and because of the very marked difference in their burning velocity and flame stability properties. APPARATUS AND PROCEDURE
Air, a t the desired pressure, was dried by passing it through anhydrous calcium chloride, and its flow was measured by using a rotameter. Technical grade propane, of about 95% purity, in which the impurities were principally ethane and isobutane in approximately equal proportions, and water-pumped hydrogen of 99.7y0 purity, were used directly from commercial cylinders. 1
Piesent address, Princeton University, Pilnoeton, N. J.
The flow of hydrogen and of propane was measured by means of calibrated capillary flowmeters. The two fuel gases and the primary air mere suitably mixed in the desired proportions and delivered to the base of burner tubes of sufficient length (35 to 100 diameters) to ensure laminar flow at the burner discharge. All tests were conducted using air-cooled burner tubes discharging to the atmosphere. The burning end of the burner tubes were lapped with No. 600 grit to ensure a smooth, flat surface perpendicular to the tube axis. For the majority of the tests, Pyrex glass No. 744 tubes 1.273, 1.093, 0.950, 0.770, and 0.573 cm. in inside diameter were used to secure the desired range of velocity gradient. For special tests on materials of construction, a stainless steel (type 304) tube 1.340 cm. in inside diameter and a transparent vitreous silica tube 1.270 cm. in inside diameter were used. With the exception of the silica tube, all had smooth, polished inner walls. The latter had the slightly wavy inner u-dl characteristic of fused silica tubing. I n order to ensure reproduribility, the following general procedure was employed:
The desired fuel gas flow was established and checked to ensure a steady state condition; primary air was introduced so that the total flow was in the region of stable flame; and the gas was ignited with a small pilot flame. The air supply was gradually altered until the desired phenomenon occurred; the flame was extinguished; and the air flow was readjusted into the region of stability. The air flow was altered in small increments toward instability, and the pilot flame applied at each setting. If flash back did not occur, the flame was extinguished and the procejs repeated until i t did. The air flow was increased in small increments until the flame began t o lift and then by an increment sufficient t o produce comIete blowoff. The blowoff was calculated from the mean of the ast two rates.
f
The foregoing procedure ensures that any instability observed is not due to a previous mixture which has failed to clear the system nor to undue overheating of the burner rim because of prolonged operation near the flash-back point where the flame is drawn into close proximity with the rim. Similarly the influence of tilted flash back or blowoff which unduly prolongs the flame existence is reduced to a minimum. In all cases, a number of determinations were made a t each set of conditions and the re-
INDUSTRIAL AND ENGINEERING CHEMISTRY
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I lop00 a000
ported values are the average of several n-hich checked wit,hin the limits of duplicability.
PRIMARY AIR (%I 100 80 60 90 TO
I 1
90W
--
RESULTS AND DISCUSSION
I ,I
ltlti/
The experimental results for air-cooled Pyrex glass No. 7 7 1 burner tubes using propane, hydrogen, and mixtures of 1he tn-o gases are presented as plots showing velocity gradient a t thcl burner wall against volume per cent fuel gas in the mixture. Velocity gradients a t the burner wall for laminar flow were calculated from the flow data using the formula:
0
7000
6000
1
5000
I
+-REYNOLDS I
,I
!Y
I I
I
NO i 2 2 m
I
I
Velocity gradient =
c
5
I
Vol. 42, No. 4
I
I
V is the total flow in the burner tube in cubic centimeter6 pci second and R is the radius in centimeters. In the present investigation only a few data were secured at Reynolds numbeii above 2200 in the region of transition flow, and were insufficient to justify extension of the curves into the turbulent flow region Bollinger and Williams ( 1 ) have shown, however, that such ti11 extension results in satisfactory correlations when the formula foi velocity gradients under turkiulent flow condition.: is employed. Experimental data for blowoff and flash back using propan(, as fuel gas are shown in Figure 1 together with results for blowoff determined by Bollinger and IIXliams ( 1) using watei-cool(,tl stainless steel tubes. Similar data using hydrogen as fuel gas : I I ~ shown in Figure 2 together with results reported by von Elbe aiitl illentser ( 2 )using water-cooled chemically resistant tubes. It nm1 be seen from a n examination of these figures that the results obtained in the present investigation using air-cooled burner tubw agree - reasonably well mith those of other investigators usiiig water-coolcd burner tubes of different size ranges and materials. This is further confirmation of the validity of the Lewis and von Elbe ( 3 ) concept that velocity gradient a t the burner wall is the critical factor in determining flame stability. The percentages of primaw ail introduced in gas-air mixtuies
100% PROPANE LEGEND BURNER TUBE DIAMETER, CM
0-0572 0 1093 10-1273
-
FUEL GAS IN MIXTURE
4v
TR
(%I
I
Figure 1. Critical Velocity Gradients a t Boundary of Gas Stream for Flash Back and Blowoff of Propane Flames in Cylindrical, Air-Cooled. Pvrex Glass hTo.774 Burner Tubes 'ofbifferent Diameters
PRIMARY AIR (%)
100%HYDROGEN
FUEL
Figure 2.
GAS
IN
MIXTURE
(%)
Critical Velocity Gradients at Boundary of Gas Stream for Flash Back and Blowoff of Hydrogen Flames i n Cylindrical, Air-Cooled, Pyrex Glass No. 774 Burner Tubes of Different Diameters
INDUSTRIAL AND ENGINEERING CHEMISTRY
April 1950
693
300
BLOW OFF FOR PROPANEHYDROQEN MiXTURE FULL BAS IN M I X T U R E (961
Figure 3. Critical Velocity Gradients at Boundary of Gas Stream for Blowoff of Flames of Propane, Hydrogen, and Various Binary Mixtures of Hydrogen and Propane
were calculated from the flow data and are shown on Figures 1 and 2. These calculations were made assuming that the technical grade propane required a n amount of primary air equivalent to pure propane, and that the hydrogen was pure. The error introduced by these assumptions is relatively small. BLOWOFF RELATIONSHIPS. The influence of increasing the hydrogen content of the fuel gas may be seen from a comparison of the blowoff curves of Figure 3. As the hydrogen content increases, there is a progressive shift toward the region of richer fuel gas mixtures. The shift is rather small for low concentrations of hydrogen, but is very pronounced at low concentrations of propane. In general, the blowoff data are easily reproducible, fall on smooth curves, and show only a minor influence of burner tube size. For the high hydrogen content mixtures and for hydrogen alone, the reproducibility is not as satisfactory as a t low hydrogen contents. Similarly, a t the lower velocities for any given burner tube, the points usually fall slightly off the mean curve. Although these deviations are not sufficient to detract from the general correlation, they do suggest that flow conditions at the burner lip are not strictly laminar a t the lower velocities. This phenomenon has been observed and discussed by the previous investigators. With 100% primary air in the fuel gas-air mixture, blowoff occurs in the laminar flow region only up to a hydrogen content in the fuel gas of about 70%. At lower percentages of primary air, virtually all the blowoff data fall in the turbulent region. For this reason only part of the blowoff curve for various propane-hydrogen mixtures is shown in Figure 4 for the fuel gas-air mix-
Figure 4. Critical Velocity Gradients at Boundary of Gas Stream for Flash Back and Blowoff of Flames for Various Binary Mixtures of Propane and Hydrogen at 100% and '70% Primary Air in Fuel Gas-Air Mixture
FUEL GAS IN MIXTURE
(%I
Figure 5. Critical Velocity Gradients a t Boundary of Gas Stream for Flash Back of Flames for Propane, Hydrogen, and Various Binary Mixtures of Hydrogen and Propane
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
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V
- 1.3qO , 3 0 4 STAINLESS
STEEL
aoo.
I
2
FUEL
3 GAS IN
4
S
6
7
M I X T U R E 1%
Figure 6. Critical Yelocity Gradients at Boundar? of Gas Stream for Flash Back and B ~ o w o f fof Propane Flames for Various ‘rjpes of CJlindrical, 4ir-Cooled Burner Tubes of Comparable Diameter
tures a t stoichiometric (lOOyG) and 70% primary air. The cffect of hydrogen addition is nonlinear. The influence upon flash back FLASH-RACK RELATIONSHIPS. of the addition of hydrogen to propane is shown in Figure 5 t,o be even more pronounced than in the case of bloivoff, and the data are somewhat less reproducible. The peaks of the flash-back areas represent regions of marked instability where a considerable scatter of experimental points is to be expected. Again the reproducibility of data is more difficult a t higher hydrogen contents. The influence of low velocity in the burner tube is more pronounced in the flash-back curves than for blovioff, especially in the region of rich gas-air mixtures. ilt lon- velocity, there is a definite tendency for flash back to occur a t lo\-rer fuel gas concentrations in the fuel gas-air mixt,ure than would be expected on the basis of over-all results. Since flash-back behavior is more dependent upon conditiolis a t the bu~rierlip than is blon-off these result,s are not unexpected.
Vo!. 42, No. 4
FLA~IE STABILITY.From the viewpoint of practical burner operat’ion, the data in the range between 60 and 1 0 0 ~ primary o air are of greatest interest’. Pressure burners frequently operatc xvith substantially 1 0 0 ~ primary o air in the fuel gas--air mixture while at,mospheric burners usually operate in the range of 60 to SO% primary air. The available adjustment between flash back and blowoff is in some cases a significant factor. I n Figure 4, the area between the flash-back and blowoff curves for 100% primary air represents t’he region of stable flames for various propane-hydrogen mixtures bu ned with stoichiometric quantities of primary air. It is readily apparent that the addition of hydrogen to propane has a marked effect in expanding the velocity gradient over which the burner will operate satisfactorily. At lower percentages of primary air, the beneficial effect of hydrogen additions is even more marked because, for any given fuel gas composition, the blowoff curves occur a t progressively higher velocity gradients while the flash-back curves occur a t progressively lower velocity gradients. IXFLCESCE OF BCRNERTUBEMATERIALS. Figure 6 shows the blowoff and flash-back curves for three air-cooled burner tubes of different materials, using propane as fuel gas. I i t h i n the limits of experimental error, the results for blowoff all fall on a single curve. This result might be expected since the phenomenon occurs above the rim of the burner. The experimental data for flash back when using the Pyrcr glass h-o, 774 tube fall on a smooth curve, but the data for t h e stainless steel and vitreous silica tube, for the most part, can be represented by another curve which appears to be shifted slightly into the region of richer €uel gas mixtures. Since the latter materials represent the extremes in heat conductivity, the result’ can hardly be explained on the basis of localized temperature effects. Although the deviations wi-ith different tube materids are of a low order of magnitude and do not detract from the over-all correlation, the result is interesting and suggests that, further work on materials of construction might be of interest, in interpreting the chain-breaking mechanism a t the burner wall. LITERATCRE CITED
(1) Bollinger, L. >I., and Williams, D. T., AratZ. B d v i s e ~ yCo??wr. Aeronaut, Tech. Note 1234 (June 1947). ( 2 ) Elbe, G. von, and Mentser, Si.,J . Chem. Phys., 13, 89 (1945). (3) Lewis, B., and Elbe, G. von, I h i d . , 11, 75 (1943). RECEIVED November 5 , 1040. Presented before the L)ivision of Gui. and Fuel Chemistry at the 116th Meeting of the AMERICAK C H E M I C ~ I . SOCIETY,Atlantic City, K. J.
Amino Acid Content of Evaporated Milk on rolonged Storage A . Z. HODSOY P e t M i l k Company, Creencille, I l l . Evaporated inilk is a leading canned food. More cans are used for i t than for an? other individual food product, and in 1947 nearly 25q. of all the Lin cans used for food were used for evaporated milk. For this reaeon and because evaporated niillc is customarily stored for short period3 before consumption, amino acid losses during prolonged storage were investigated. Of the seventeen amino acids studied significant losses were found for on13 lysine ( 1 7 7 ~ ) ,
histidine (17%), and arginine (11 %) during 5 j ears’ s torape. During 15 to 17 months’ storage, which is longcc than the customary storage period, losses of these thrce amino acids were not greater than 49% in any case.
T
HE amino acids in the protein of foods are not aluays
(111-
tirely stable during prolonged storage. Henry and Icon (3) leAxxtedthat the biological value of the proteins of dly shim mlli
