Combustible Vapor-Measuring Instruments USE ON SOLVENT MIXTURES GEORGE B. 1\IORNIIC Minnesota Mining & Manufacturing Cornpan-y, S t . Paul, .Minn.
I
S S T R U N E S T S eniployThe formulas on niiscd .onditionsof diffusion. Diffusion ill have less opportunity promote its full influence when the air over the evaporating sur. T ~ ~ R B V L E.%IR S T EVAPORATIOS. In order.
!I+
rest thr: validity of the formulas, equal parts of methyl and isw
propyl alcohols by volume were mixed. In keeping with the prillciples of ideal mixtures the total volume iyas equal to the sum ot rhe t w o separate volumes. Eight hundred milliliters of the niislure werv transferred t o a 1-liter beaker set in a constant temperature bath. The mixture was agitated continuously at a slow rat(. af speed. Air a t a constant velocity was passed through a 1-incli insidc diameter rubber hose located in such a position that the air jet from the hose impinged downward at a 45' angle againsr rhc opposite inside wall of the beaker. This arrangement allonid an abundance of turbulent vapor free air to pass over the evaporating auiiace of the liquid in fulfillment of the conditions of thitypes of evaporation. A thermometer was immersed in the solvent mixture and by controlling the temperature of the bath a constant. rpmpcrature was maintained in the solvent nlixturc. iritcrvals during the evaporation process an analysis u i the ire \vas made by means of t,he refractive index. Figure 3 a comparison of the curves between the experimentally dewrmincd and calculated values for a n evaporating temperature of 25" C. When an exponent i = 0.56 was used, as was previously suggested for vapor free turbulent air, the experimental and calculatcd curves were identical except for a very slight dcviation in the region between 100 and 250 ml. total volume. To .holy the maximum deviation possible in the formula, i W R - :+-signwl t h r
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 41, No. 10
Assuming that the effects of the components on the indicator are additive:
R
=
+ R2 t Ra
I?,
(301
From Equation 1:
+ S?(p.p.m.?)+ Ss(p.p.m
K = S1(p.p.m.l)
(31)
1)
Substituting values of p.p.m.2and p.p.m.3 from Equations 27 and 28 and rearranging: (32 1
Similarly :
Equations 32, 33, and 34 show the concentration of each component in the vapor phase when the instrument reading is given. The equations for a two-solvent mixture are: (35)
600
400 TOTAL VOLUME IN ML.
zoo
0
Figure 4. Comparison between Calculated and Experimental Data for Still Room Air Evaporation of a Jlixture of Alcohols
Any number of components may be treated in a similar manner. DETERMINING CORRECTIOS FACTOR F
Case IV, Constant Evaporating Mixtures. Vhen a mixture of solvents possessing constant boiling properties a t a certain coinposition is subjected to evaporation it may exhibit a point of maximum (or minimum) vapor pressure, such as it does for constant boiling conditions. The composition a t which this O C C U I S during evaporation will be different from that during boiling. Determination of the vapor phase ratios for this type of qolvent mixture is complex, and recourse to the specific literature and data for the particular solvent combination under consideration is advised. King and Smedley (12) ascertain the composition a t which constant evaporating properties occur by means of the refractive indeu. Specific data for some mixtures of this type may be found from tables in Hofmsnn (8) and International Critical Tables (IO).
’
F is the factor with which to multiply the observed readiiig iri order to get the true reading in per cent of the l o m r explosive limit. If the ratios between the components are known and the assumption is made that Le Chatelier’s rule holds it is possible to compute the lower explosive limit and the factor F . First, it will be necessary to find the per cent by volume E that each component comprises of the total vapor volume only, as defined by Equation 38. (37)
100
l00n
1
= -
(30 ’
Similarly:
I t this point in the development of the method it is assumed that the ratios between the components in the vapor phase are knovin. By using these ratios in conjunction with the observed instrument reading a method will now be developed for calculating the actual Concentration of each component in the vapor air mixture. Denoting the identity of the component by a numerical subscript and letting the component ratios in the vapor phase of a three component mixture be expressed by g, h, and j :
100iz -
E?
CALCULATISG CORCENTRATION O F COIIPOhENTS FROM INSTRUAMENTREADINGS
7!
lO0n
E)
__
The lover esplosive limit of the misture is now found by Le Chatelier’s rule ( 1 1 ) which is given by: L.E.L.
100
=
,
L.E.L.1
(421
1::j
L.E;.L.r
+
L
X
L.E.L.], L.E.L.2, and L.E.L.3 are the recognized lower explosive limits of the components in pure form. The parts per million of the first component a t the lower explosive limit of the mixture will be: p,p.ni.l a t L.E.L.
=
El (L.E.L.) 100
(43)
P.p.m.a or p.p.m.3 could he used; one should choose the value that will give the greatest accuracy. (Note that 10,000 ’ X % = (per cent by volume) = p.p.m.; therefore, 100 X % P.P.m.)
INDUSTRIAL AND ENGINEERING CHEMISTRY
October 1949
Substitute the p.p.m.I obtained from Equation 43 in Equation 3 2 and solve for R, then: Factor F =
100
R at the L.E.L.
(44)
The corresponding equations for a two-component mixture are :
L.E.L. =
100
E,
Determine the p.p.m.1 from Equation 43 and substitute this value in Equation 33 to get R, then find the factor from Equation 44. If the components of a mixture evaporate under the conditions of case I, it is convenient to use Le Chatelier's rule by substituting mole fractions for per cent by volume concentrations, calculating the lower explosive limit directly, and then proceeding a.s previously outlined. =
lOO(Xj
L.E.L.=
100 lOO(1')
~
arise with respect to the difference in evaporation rates between the components of the fraction. The method of attack for the problem under consideration then becomes one involving two types of experimental determinations. The first of these is 011viously an investigation of the effect of varying the evaporating temperature while the second is the control investigation in which the evaporating temperatures are held constant and sufficiently high to ensure practically instantaneous and complete evapor:ition. By this means it is intended to shoLv the practical effect of incomplete evaporation on the lorver limit. In reference to Table I the boiling ranges of four commercially available petroleum fractions are listed as *i,B, C , and D, each from a different vendor. The first of these, designatod by the letter is the one upon which the determinations were madr; the others are included to shov the similarity between some of the available varieties.
(47)
E9
L.E.L.l+L.E.L.r
L.E 1,
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lOO(Z, L.E.L.,
TABLEI . BOILISGR.LSC,ESASD FLASH POISTSOF Sqvt: P E T R O L EFR.XTIOSS U~I Fraction
B
A
C
11
(-181
EXPERI3IESTAL METHOD FOR D E T E R I I I S I S G FACTOR F
Frequently, circumstances arise whereby i t becomes imperative to predict with some degree of certainty the flammable characteristics of petroleum fractions possessing a boiling range in the neighborhood of 300" to 400" F. Sotwithstanding the difficulty of igniting these fractions a t ordinary room temperatures in comparison t o those of a lovier boiling nature, elevations of temperatures equivalent to those usually encountered in industrial w x m ing and drying operations may influence the behavior of these fractions sufficiently to Tvarrant additional conzideration n-ith respect to their flammability. The application of combustible vapor-measuring instruments for determining the extent of these flammable characteristics calls for a knowledge of the lower esplosive limit a t the various elevated temperatures being encountered. Equation 44 may be used for determining t h e factor when the lower explosive limit is ascertained. Examination of a commercial petroleum fraction of boiling range 332" to 393' F. was undertaken because data pertinent to its flammability were unavailable. Furthermore, this particular fraction would afford an excellent opportunity to stud>-the flammable characteristics a t various evaporating temperatures and conditions. Coward and Jones (93j give 1.3% for the lower and 4.970 for the upper limits of flammability for the vapors arising from a crude petroleum of boiling range 142" to 596" F. This comprises a much n-ider range than the fraction to he examined and does not consider variations in evaporating temperatures. Coward and Jones ( I ) are of the opinion that for ordinary increases in temperature the value of the lower flammable limit does not vary much and shon- that the higher the temperature the lower the value of the lower flammable limit. Dufour and Matson (3)found the lower limits of flammability of methyl ethyl ketone vapor in air t o be 1.7, 1.5, and 1.3Y0 by volume a t initial temperatures of 212", 302", and 392" F., respectively. The corresponding upper limits ryere found to be 9.7, 9.8, and 9.9%. Usually, in a solvent mixture i t is possible to calculate the approximate lower limit by means of Le Chatelier s rule when the definite composition is known. However, in dealing with petroleum fractions such information is difficult to obtain in view of the complexity of these fractions. Additional complications
I n respect to composition. it was necessary to determine the values for fraction -1. The values for the other iractions werc available from their respective vendors. Here again. there is considerable similarity except for fraction D, which seemingly has undergone an extractive treatment for the removal of aromatics and olefins.
T ~ B L11. E CO~IPOSITIOS Fraction
A
B
C
D
I n order to prepare predetermined concentrations of vapor-air mixtures of fraction .i i t n-as necessary to find the average molecular n-eight. This lvas obtained by a modification of the Victor Jleyer method for high boiling liquids and was found to be 142. The method of determining the lower limit n-as similar to that employed by Jones ( I 2 j except that the equipment was electrically heated and controlled throughout t o maintain any desired experimental temperature between that of the room and 400" F. The explosion chamber consisted of a vertical Pyrex tube 4 feet long and 2 inches in diameter. The bottom end was ground flat to fit a removable glass cover plate so as to provide a convenient means of access to the chamber for purging or ignition purposes. An 8-mm. inside diameter Pyrex glass tube was sealed to the top and curved to run donm the outside of the chamber to within 6 inches from the bottom. The outside of the chamber nas spirally vound with Nichrome heater wire. I n order t o obtain different chamber temperatures the heater wire was connected to a variable transformer. A thermocouple was mounted within the central portion of the chamber and connected to a potentiometer so that chamber temperatures could be indicated. T o prevent heat loss, the chamber and S-mm. tube were insulated from top t o bottom except for a 0.5-inch wide observation strip for viewing the propagation of the flame the full length of the chamber.
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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
Vol. 41, No IO
and explosion chamber temperamu-es Bere practically identical The values given in Tahlr TIT are plotted as a dotted line curve irt Figure 5 .
TABLE
111.
LOT5 E R
EXPLOSIVE LIMIT
.4T DIFFEREVT
TEVPERATURES G a r h r r t o r Block Temp., F
Ghdrubi,r Temp., * 3.
185 250 300 $00
187 250 300 100
100 100 400
126 I58 177 223 277
400 400
L 700
I@*
EXPLOSION C Y ? '/sSF*
% r ~
0.972 0 833 0 750 0.730
1 00
0 ')I2 0 888 0 834 0 748 0,730
-
i _
3co F W P , * k.
4M,
Figure 5 . Comparison of Lower Explosive Limit Values for Different Kvapnrating Temperature-
.*
-
Ano
400
70
L.E.L.(% Vol
continuous source of adjustable vapor concentr:itic!rJ:, i n X I : was obtained from an automatically controlled and coinpletrl~ heated vapor mixer. This was accomplished by passing the ire? room air through a rotameter and into an electrically heatcd arid controlled preheating chamber. From there it, passed into a carburetion block where i t was mixed Kith the liquid fractiori 4 injected from a uniformly driven syringe carefully calibrated SUP to ensure delivery of the liquid into the carburetion block at a uniform rate, The carburetion block was packed with copper turn. ings t o form a large mixing surface. The temperature of the piliburetion block Ras controlled electrically. In order to ensure adequate mixing the heated v a p c r - m 11ii1ture was passed t,hrough another heated mising block fillrd witif copper turnings. From there t,he misture passed t,hrough 3 1 1 i r ~ . sulated and heated delivery line containing a T-stopcock imoected with the &mm. delivery tube leading to the top of t h i explosion chamber. For each determination the system was carefully purged witi, the predetermined mixture aft,er first sliding the lubricated glaw plate a t the bottom of the explosion chamber to create a small opening for the escape of the vapor-air mixture. It was found b j experience that a purging of about 1 minute in duration was sufficient t o create a uniform mixture of the desired concentration. however, during the experimentation the purging time was e ~ tended to 2 minutes. T h e chamber was then closed by sliding the plate back into place. At the same time the vapor-sir .mi\Lure was directed into the atmosphere by means of th- l'-St
