pf = film pressure factor, atm. R = g a s constant,oatm. CU. ft./(lb. mole.) OR t =temperature, F. u = convective velocity, feet per second p = viscosity of air film, lb./ft. hr.
p = density of air film,
-’-
Ib
CU.
6 = reciprocal
ft.
average temperature,
1 F.
-0-
Af = fugacity potential of vapor, atm. difference between interface and bulk air, 6 = unique function defined by Equations 1 and 2
%?I = temperature
OF.
Dimensionless Groups X
001‘ 001
J I
I
2 = Prandtl number k
X ,
,
I
l
l
1
I
I
1
,
1
1
1
01
I
I
I
I
I I , i t
= Schmidt number
IO
UNIT EVAPORATION RATE, e (LB/SQ
g , @ - h m . d JPD .p’
FT HR)
= Grashof number
F i g u r e 4.Dependence of vaporization rate on fugacity potential
Pa
defining t h e p r o c e s s of h e a t t r a n s f e r from a horizontal p l a t e facing downward (7). T h e unit vaporization r a t e i s plotted a g a i n s t fugacity potential for e a c h liquid i n F i g u r e 4 and t h e d a t a can b e well correlated b y t h e following equation:
’’
g . d J . p I . k - 1 . .~
LL2
= Grashof number for mass transfer
hd
- =
k
kg. R . T.pf d
Nusselt number
+
e
=
C. Af””
(9)
where t h e v a l u e of C v a r i e s from liquid
+?
Liquid
C
Water Carbon tetrachloride Ethyl alcohol Benzene
6.58 2.96 1.80 1.33
liquid.
It i s a n t i c i p a t e d that continuation of t h i s part of t h e i n v e s t i g a t i o n under sub- and superatmospheric p r e s s u r e will provide valuable information, from which more general r e l a t i o n s can b e derived.
NOMENCLATURE Cp = h e a t capacity at constant pressure, B.t.u. per OF. D = diffusion coefficient, sq. feet per hour d = characteristic dimension, feet lb. e = unit vaporization rate, ___ hr. sq. ft. f = fugacity, atm. g = acceleration due t o gravity, ft./sec.’ h = h e a t transfer coefficient, B.t.u./hr. sq. ft. OF. k = thermal conductivity, B.t.u./ft. ‘F./ft.
kg = mass transfer coefficient, lb. moles/hr., sq. ft. atm. P = total pressure, atm.
D*P
= Nusselt number for mass transfer
9 = Reynolds number 11
LITERATURE CITED (1) Arnold, J. H., Ind. Eng. Chem. 22, 1091 (1930). (2) Boelter, L. M. K., Gordon, H. S., Griffin, J. R., I b i d . , 38, 59 (1946). (3) Buddenberg, J. W., Wilke, C. R . , I b i d . , 41, 1345 (1949). (4) Hirschfelder, J. O., Bird, R. B., Spotz, E. L., J. Chem. Phys. 16, 968 (1948). (5) International Critical T a b l e s , Vol. 111, p. 215, McGraw-Hill, New York, 1928. (6) Ibid., Vol. V, p. 1, 1929. (7) Ibid., p. 62. ( 8 ) Kranssold, H., Forsch. Gebiete Ingenieurw. 5, 186 (1934). (9) Kyte, J. R., Madden, A. J., Piret, E. L., Chem. Eng. Progr. 49, 653 (1953). (10) Perry, J. H., “Chemical Engineers’ Handbook,” p. 370, McGraw-Hill, New York, 1950. (11) Saunders, 0. A . , Proc. Roy SOC. (London) 157, 278 (1936); 172, 5 5 (1939). (12) Touloukian, Y. S., Hawkins, G. A., Jakob, M., Trans. A m . SOC.Mech. Engrs. 70, 13 (1948). (13) Wilke, C. R., Chem. Eng. Progr. 49, 663 (1953). (14) Wilke, C. R . , Ind. E n g . Chem. 42, 471 (1950). (15) Ibid., 43, 1641 (1951). Received for review February 7, 1955. Accepted August 9, 1956.
Upper Explosive Limits of Cumene JUDSON C. BUTLER’ AND WILLIAM P. WEBB California Research Corp., Richmond, C a l i f .
In
common with most other hydrocarbons, cumene forms e x p l o s i v e mixtures with air. T h e s e e x p l o s i v e mixtures a r e limited or rendered nonexplosive i n t h r e e w a y s : by diluting with a i r until t h e mixture i s t o o l e a n t o explode (lower limit), by diluting with fuel until t h e mixture is t o o rich t o ‘Deceased, March 30, 1956.
42
INDUSTRIAL AND ENGINEERING CHEMISTRY
explode (upper limit), and by diluting with a n inert subs t a n c e until t h e r e i s insufficient oxygen t o explode. Previously, only a small amount of information w a s a v a i l a b l e concerning t h e e x p l o s i v e limits of cumene (1-3), and t h i s information pertained only t o t h e limit a s a function of t h e oxygen-nitrogen-cumene ratio a t atmospheric pressure. However, i t i s impossible to predict t h e e f f e c t of VOL. 2,
NO. I
p r e s s u r e or temperature c h a n g e s upon e x p l o s i v e limits. C o n s e q u e n t l y , t h e p r e s e n t i n v e s t i g a t i o n w a s undertaken t o determine t h e e x p l o s i v e l i m i t s of dry and wet c u m e n e under various c o n d i t i o n s of temperature, p r e s s u r e , a n d oxygen content. To carry o n t h i s work, i t w a s n e c e s s a r y t o d e s i g n a n d c o n s t r u c t equipment c a p a b l e of furnishing a d e s i r e d g a s e o u s mixture a t e l e v a t e d p r e s s u r e s and of determining whether t h i s mixture w a s e x p l o s i v e . P r e v i o u s i n v e s t i g a t o r s , notably t h o s e of t h e United S t a t e s Bureau of Mines, (4, 3, h a v e s t u d i e d t h e v a r i o u s f a c t o r s a f f e c t i n g t h e l i m i t s of flammability. T h e most important f a c t o r s are: method of ignition, direction of f l a m e propagation, d i m e n s i o n s of t h e reaction v e s s e l , humidity, p r e s s u r e , temperature, and turbulence. In t h e p r e s e n t work, t h e first t h r e e f a c t o r s were h e l d cons t a n t and t h e l a s t four were i n v e s t i g a t e d .
APPARATUS An a p p a r a t u s , incorporating t h e findings and recommend a t i o n s of t h e B u r e a u of Mines and d e s i g n e d t o g i v e maximum r a n g e t o t h e e x p l o s i v e mixtures, w a s c o n s t r u c t e d . A s finally d e v e l o p e d , t h e a p p a r a t u s c o n s i s t e d e s s e n t i a l l y of f i v e parts: I. A g a s f e e d s y s t e m b y which various mixtures of t w o g a s e s could b e introduced into the s y s t e m from compressed g a s c y l inders. 2. A saturator bomb i n which the g a s stream w a s saturated with the vapor of the t e s t liquid or liquids. 3. A combustion chamber in which t h e g a s vapor w a s ignited by a spark and flammability w a s measured. 4. A condenser s y s t e m for condensing and c o l l e c t i n g the vapor from the g a s streams. 5. A constant pressure control regulator and a wet-test meter for maintaining the pressure and measuring the volume of g a s flowing through the s y s t e m . A schematic diagram of t h i s flow s y s t e m i s g i v e n in Figure 1. T h e g a s f e e d s y s t e m c o n s i s t e d of a cylinder of high p r e s s u r e nitrogen and a c y l i n d e r of high p r e s s u r e a i r , e a c h c o n n e c t e d t o a common t e e . V a l v e s i n e a c h l i n e permitted v a r i a t i o n s in t h e flow of t h e t w o g a s e s , supplying v a r i o u s mixtures. T h e mixed g a s e s w e r e p a s s e d through a s m a l l orifice (a n e e d l e valve) t o provide further mixing. G a g e s were a v a i l a b l e t o m e a s u r e t h e p r e s s u r e drop a c r o s s t h i s v a l v e . In p r a c t i c e , t h i s p r e s s u r e drop varied from 100 t o 4 0 0 pounds per s q u a r e inch gage. Before t h e mixed comp r e s s e d g a s p a s s e d i n t o t h e s y s t e m , a s m a l l amount w a s continuously a n a l y z e d by a P a u l i n g oxygen a n a l y z e r . T h e high p r e s s u r e g a s mixture t h e n p a s s e d through a double v a l v e s y s t e m i n t o t h e s a t u r a t o r bomb. T h e g a s w a s piped t o t h e bottom of t h e bomb, where i t p a s s e d through a fritted s t e e l p l a t e and bubbled up through t h e t e s t liquid. T h e temperature of t h i s liquid w a s maintained by a n e l e c t r i c heater, wound around t h e bomb. T h e temperature of t h e liquid w a s m e a s u r e d by t h r e e thermocouples l o c a t e d a t various d i s t a n c e s from t h e bottom of t h e saturator. Add i t i o n a l temperature r e a d i n g s w e r e made of t h e bomb c a s i n g by a t t a c h e d thermocouples. T h e p r e s s u r e and temperature of t h i s s a t u r a t o r determined t h e mole per c e n t of t h e liquid i n t h e outgoing vapor. From t h e saturator, t h e vapor p a s s e d through h e a t e d l i n e s i n t o t h e combustion chamber. T h i s v e s s e l w a s also a p r e s s u r e bomb, 2.5 i n c h e s in diameter and 32 i n c h e s long, h e a t e d t o a temperature 1 0 ' F . above t h e s a t u r a t o r temperature t o prevent c o n d e n s a t i o n . Connected t o t h i s bomb w e r e a drain v a l v e , a rupture d i s k , a maximum p r e s s u r e i n d i c a t i n g g a g e , and a n inlet valve. About 8 i n c h e s a b o v e t h e bottom, a spark plug w a s i n s e r t e d in a n opening in t h e s i d e of t h e v e s s e l . T h e t w o e l e c t r o d e s of t h e s p a r k plug w e r e c o n n e c t e d by a s m a l l p i e c e of platinum wire (B. & S. g a g e No. 36). A s m a l l amount of gun c o t t o n w a s a l s o wound around o n e of t h e e l e c t r o d e s . T h e e l e c t r i c a l connection t o 1957
Figure
1.
Schematic diagram of e x p l o s i v e l i m i t s apparatus
t h e s p a r k plug w a s regulated f r o m i n s i d e t h e control room. E i g h t e e n i n c h e s a b o v e t h e s p a r k plug well t h e r e w a s another o p e n i n g i n which w a s i n s e r t e d a plug holding a collodion s t r i p . If t h i s s t r i p of collodion w a s burned during a n experiment, t h e t e s t mixture w a s c o n s i d e r e d e x p l o s i v e . T h e temperature of t h e vapors i n t h e combustion chamber w a s measured by a thermocouple l o c a t e d i n t h e c e n t r a l thermowell. T h i s s a m e thermocouple measured t h e temperature r i s e occurring during a n explosion. After l e a v i n g t h e combustion chamber, t h e vapor p a s s e d through a 4-foot water-cooled c o n d e n s e r , where t h e vapori z e d liquid w a s c o n d e n s e d and c o l l e c t e d i n a trap. An outlet n e e d l e v a l v e on t h i s t r a p permitted emptying, e v e n during a run or when operating under high p r e s s u r e . From t h e c o n d e n s e r s e c t i o n t h e g a s s t r e a m p a s s e d through another double-valve s y s t e m into t h e control room where t h e r e w a s an automatically controlled c o n s t a n t press u r e valve. T h i s valve maintained t h e s y s t e m a t any des i r e d p r e s s u r e up t o 150 pounds per s q u a r e inch gage. T h e e x i t g a s from t h i s v a l v e w a s a t atmospheric p r e s s u r e a n d w a s measured by a wet-test meter. T h e e s c a p e g a s e s w e r e then vented t o t h e atmosphere. B e f o r e venting, t h i s g a s could b e a n a l y z e d for oxygen c o n t e n t if d e s i r e d . All t h e c o n t r o l s n e c e s s a r y for regulating t h e a p p a r a t u s during an experiment w e r e l o c a t e d in a control room. T h e r e s t of t h e a p p a r a t u s w a s l o c a t e d within a c o n c r e t e high p r e s s u r e cell. T h u s , i t w a s not n e c e s s a r y for t h e operator t o b e n e a r t h e bombs during high p r e s s u r e and ignition. A s y s t e m of mirrors permitted t h e operator t o view t h e outs i d e p r e s s u r e g a g e s from t h e s a f e t y of t h e control room.
EXPERIMENTAL PROCEDURE T h e s a t u r a t o r bomb w a s charged with 2000 t o 3000 ml. of t e s t l i q u i d , which c o n s i s t e d of c u m e n e or cumene a n d water. P h e n o l , 0.05 weight % of cumene, w a s added t o prevent oxidation. While t h i s material w a s coming u p to temperature, a fresh s p a r k plug and collodion s t r i p w e r e i n s e r t e d in t h e combustion chamber. T h e a p p a r a t u s w a s then pressurized with t h e t e s t g a s which w a s allowed t o flow through t h e s y s t e m until a dynamic equilibrium w a s reached. At equilibrium c o n d i t i o n s , t h e combustion chamber w a s s w e p t out with s i x t i m e s i t s volume of t e s t mixture. After t h e sweepout time, t h e t e s t mixture w a s a n a l y z e d by measuring t h e amount of liquid c o l l e c t e d in t h e t r a p w h i l e a c e r t a i n volume of g a s p a s s e d through t h e apparat u s . T h i s a n a l y s i s w a s compared with t h a t predicted from t h e temperature and p r e s s u r e of t h e s a t u r a t o r . When t h e s e two v a l u e s a g r e e d , t h e saturator, firing chamber, and cond e n s e r w e r e i s o l a t e d and t h e s p a r k plug w a s a c t i v a t e d . E x p l o s i o n s w e r e indicated by a temperature a n d p r e s s u r e r i s e . However, t h e final criterion w a s t h e condition of t h e collodion s t r i p placed near t h e t o p of t h e chamber. A t e s t w a s c o n s i d e r e d a n e x p l o s i o n if t h e collodion s t r i p w a s CHEMICAL AND ENGINEERING D A T A SERIES
43
~
IG"
~
0
EXPLOSION NO EXPLOSION
100
cc . S80
I
0 w. W
O
L
a a v) VI
2
60
1
I
I
EXPLOSIVE REGION
l
I
a.
-
! I I
I
I
I
Y
Il
I
I
920
140
160
180
200
220
240
280
280
300
SATURATION TEMPERATURE, 'F. Figure
2. E f f e c t of temperature and pressure upon upper explosive l i m i t s of cumene and a i r
completely burned, on t h e limit if i t w a s partly burned, and a nonexplosion if it w a s i n t a c t . T h e following experimental d a t a were c o l l e c t e d on e a c h run: Temperature of saturator contents (three points) Temperature of inside of combustion chamber Pressure Rate of g a s flow at atmospheric pressure Oxygen content of entrance and exit g a s e s Milliliters of liquid collected from condenser trap Cubic feet of g a s p a s s e d while liquid s a m p l e s were collected Pressure r i s e (if any) Temperature rise (if any) Condition of collodion strip after ignition
DISCUSSION OF RESULTS Calibration Runs. Before work on t h e cumene s y s t e m s , w a s begun, t h e r e s u l t s o b t a i n a b l e by t h e present a p p a r a t u s were compared with t h o s e obtained from other a p p a r a t u s , by determining t h e upper e x p l o s i v e l i m i t s of air-benzene a t normal p r e s s u r e a n d of air-propane at high p r e s s u r e . T h u s , i t w a s found t h a t , at atmospheric p r e s s u r e , t h e upper limit of b e n z e n e mixtures occurred a t 7.3 mole % of b e n z e n e . T h i s c o m p a r e s favorably with t h e a c c e p t e d v a l u e of 7.1 mole % of b e n z e n e (5). At 100 pounds per s q u a r e i n c h g a g e , t h e upper limit of propane mixtures occurred between 22.0 and 2 3 . 7 mole % of propane, w h e r e a s t h e Bureau of Mines (6) v a l u e i s 25.0 mole % of propane. Cumene-Air System. Initially, t h e e f f e c t of p r e s s u r e upon t h e upper e x p l o s i v e l i m i t s of t h e cumene-air s y s t e m w a s s t u d i e d . F i g u r e 2 is a plot of t h e bracketing v a l u e s a t s e v e r a l temperatures and p r e s s u r e s . T h e a p p a r a t u s furnished s a t u r a t e d mixtures only; therefore, in t h i s graph t h e temperatures a n d p r e s s u r e s a r e t h o s e of t h e saturator bomb. T h e a r e a t o t h e left of t h e c u r v e in F i g u r e 2 is t h e region of e x p l o s i v e mixtures, a s determined by s a t u r a t i o n p r e s s u r e and temperature. T h i s region e x t e n d s until a lower limit ( e x c e s s oxygen) i s reached a t a lower temperature. However, t h i s lower limit w a s not i n v e s t i g a t e d i n t h e present study. At atmospheric p r e s s u r e a n d s a t u r a t i o n temperat u r e s above 174'F., t h e vapor i s t o o rich i n cumene t o explode; a t 100 pounds per s q u a r e i n c h g a g e , t h e s a t u r a t i o n temperature n e c e s s a r y t o obtain a fuel-rich, nonexplosive mixture is 295'F. T h e s e v a l u e s correspond t o mixtures containing 8.8% of cumene-91.2% of a i r a t atmospheric p r e s s u r e and 10.8% of cumene-89.2% of a i r at 100 pounds per s q u a r e i n c h gage. Several intermediate v a l u e s were a l s o obtained and a r e i n d i c a t e d i n F i g u r e 2. T h e s e r e s u l t s c a n a l s o b e e x p r e s s e d a s t h e amount of s a t u r a t i o n p r e s s u r e n e c e s s a r y t o obtain an e x p l o s i v e mixture a t a given saturation temperature. T h u s , at 200'F. a p r e s s u r e of 1 2 . 5 pounds per s q u a r e i n c h gage i s n e c e s s a r y to obtain an e x p l o s i v e mixture; w h e r e a s , a t 298'F. a pres-
44
INDUSTRIAL AND ENGINEERING CHEMISTRY
s u r e in e x c e s s of 108 pounds per s q u a r e inch gage is n e c e s s a r y . In a l l c a s e s , t h e v a l u e s given a r e valid only for s a t u r a t e d vapor mixtures. Cumene-Air-Water System. T h e s e dry cumene experiments gave a n upper e x p l o s i v e limit v a l u e at various p r e s s u r e s a s a b a s e c u r v e for further work. However, a s water'narrows t h e explosive limits of many hydrocarbons, a s t u d y t o determine t h e e f f e c t of water upon t h e upper e x p l o s i v e limits of cumene w a s undertaken. B e c a u s e of t h e d e s i g n of t h e equipment, only mixtures s a t u r a t e d with both water and cumene w e r e obtainable. F i g u r e 3 i s a plot of t h e bracketing v a l u e s a t s e v e r a l temperatures and p r e s s u r e s . A s u s u a l , water lowered t h e upper e x p l o s i v e limit of cumene-air mixtures from t h e v a l u e s obtained with dry cumene a n d air. For example, a t atmospheric p r e s s u r e a s a t u r a t i o n temperature a b o v e 1 7 4 ° F . i s n e c e s s a r y t o render a dry cumene-air mixture nonexplosive; with w e t cumene mixtures, a temperature of only 1 4 0 ° F . or above i s n e c e s s a r y t o g i v e a nonexplosive mixture. Again, i n c r e a s e d p r e s s u r e r a i s e d t h e upper e x p l o s i v e limit. F o r example, a t atmospheric p r e s s u r e , t h e limiting mixture contained 3.9% of cumene; but, at 70 pounds per square inch gage, t h e v a l u e i n c r e a s e d t o 5.7% of cumene. E x p r e s s e d in terms of s a t u r a t i o n temperature and p r e s s u r e , a wet vapor mixture below 1 4 0 ° F . will explode at a t m o s pheric pressure, w h e r e a s a t 2 1 4 ° F . a pressure in e x c e s s of 50 pounds per s q u a r e inch g a g e i s n e c e s s a r y t o form a n e x p l o s i v e mixture. T h u s , t h e p r e s e n c e of water i n c r e a s e s t h e s a f e t y of t h e cumene oxidation by narrowing t h e region of e x p l o s i v e mixtures. Ignition under Flow Conditions. B e c a u s e turbulence a f f e c t s t h e flammability of a g a s e o u s mixture, comparative runs were made on a s t a t i c and dynamic s y s t e m . It w a s found that, a t 238'F. a n d 85 pounds per s q u a r e inch g a g e and with t h e air flowing at t h e linear r a t e of 0.1 foot per s e c o n d through t h e combustion chamber, a mixture c o n s i s t ing of 5.8% of cumene, 28.4% of water, and 65.8% of a i r would explode, although mixtures of t h e s a m e composition were nonexplosive under s t a t i c conditions. T h e temperature and p r e s s u r e r i s e indicated t h a t t h i s mixture w a s very c l o s e t o the dynamic upper limit. However, t h i s experiment d o e s i n d i c a t e that t h e upper e x p l o s i v e limits a r e i n c r e a s e d slightly by flow. Diminished Oxygen Systems. T h e foregoing work on explosive limits of cumene-air s y s t e m s indicated a need t o know more about t h e e f f e c t of oxygen on t h e explosive limits of cumene. Consequently, determinations were made of t h e e x p l o s i v e limits of both dry and wet cumene under conditions of diminished oxygen content. In t h i s manner, t h r e e component diagrams ( F i g u r e s 4 and 5 ) of t h e e x p l o s i v e region were obtained both at atmospheric p r e s s u r e a n d at 80 pounds per s q u a r e inch gage. e o
EXPLOSION NO EXPLOSION
I
'
I
z 0
5
40'
I
/I
T3
s 5
ro
20
0
I20
140
160
I
I
I
180
200
II
1
i
NONEXPLOSIVE REGION REGION
I
I 220
II
I
240
260
280
300
SATURATION TEMPERATUFL'E F i g u r e 3. E f f e c t o f temperoture and pressure upon upper explosive l i m i t s o f cumene, air, and water
VOL. 2,
NO. I
EXPLOSION NO E X P L O S I O N I N T E R POL A T E 0 V A L U E (FROM F I G . 2)
MOLE PERCENT
A
MOLE PER C E N l
MOLE PER CENT OXYGEN Figure
4. E x p l o s i v e l i m i t s of dry cumene-oxygen-nitrogen
T h e c u r v e s i n F i g u r e s 4 a n d 5 a r e upper limit c u r v e s only, e x c e p t for t h e one at 80 pounds per s q u a r e i n c h g a g e in t h e wet s y s t e m . In t h i s c a s e enough d a t a w e r e c o l l e c t e d t o plot t h e e n t i r e e x p l o s i v e a r e a , bounded by both t h e upper and lower limit rurves. In t h e remaining c a s e s , t h e upper limit c u r v e w a s c a r r i e d only t o t h e turning point a s shown. P r e s u m a b l y t h e lower limit boundary is a straight l i n e parallel t o t h e left-hand e d g e of t h e triangular diagrams a n d of t h e s a m e cumene concentration a s t h e turning point. However, t h i s region w a s not examined. F i g u r e 4 i s a ternary diagram of t h e e x p l o s i v e l i m i t s of t h e s y s t e m cumene-air-nitrogen. T h e inner curve is t h e upper limit of t h e e x p l o s i v e region a t atmospheric p r e s s u r e . T h e s h a r p break in t h i s c u r v e at about 4.0% of cumene i s surprising and may b e d u e t o a difference in t h e n a t u r e of t h e oxidation reaction a b o v e and below t h i s point. At a t mospheric p r e s s u r e , dry g a s e o u s mixtures containing l e s s than 10%oxygen a r e n o longer e x p l o s i v e , r e g a r d l e s s of t h e cumene concentration. T h e outer curve of F i g u r e 4 is t h e upper limit of t h e e x p l o s i v e region at 80 pounds per s q u a r e inch gage. A s expected, t h e e x p l o s i v e a r e a a t high p r e s s u r e is greater than t h e e x p l o s i v e area a t a t m o s p h e r i c p r e s s u r e . T h e minimum oxygen required for combustion a t 80 pounds per s q u a r e inch g a g e w a s only 8.5%. F i g u r e 5 is a ternary diagram of t h e e x p l o s i v e l i m i t s of t h e s y s t e m cumene-oxygen-nitrogen-water a t t h e s a m e two p r e s s u r e s . In t h e s e c a s e s t h e two inert s u b s t a n c e s (nitrogen and water) a r e plotted a s one component, although only t h e nitrogen i s independent. T h e molar amount of w a t e r i s directly proportional t o t h e molar amount of cumene. B e c a u s e of t h i s , F i g u r e 5 i s valid only for s y s t e m s s a t urated with both water and cumene a t a given temperature and p r e s s u r e . T h e inner c u r v e is t h e upper limit of t h e e x p l o s i v e region at a t m o s p h e r i c pressure. T h e a r e a within t h e outer curve i s t h e e x p l o s i v e region a t 80 pounds per s q u a r e inch gage. T h e s e t w o c u r v e s c l e a r l y i l l u s t r a t e t h e widening of the e x p l o s i v e limits t h a t t a k e s p l a c e with a n i n c r e a s e i n p r e s s u r e . T h e over-all e x p l o s i v e a r e a of t h e s e wet mixt u r e s i s somewhat l e s s than t h a t of the dry mixtures (Figure 4). A t a t m o s p h e r i c p r e s s u r e , t h e minimum amount of oxygen n e c e s s a r y for combustion i s 11.5%, w h e r e a s only 9.4% i s required a t 80 pounds per s q u a r e inch gage. T h e s e minimum oxygen v a l u e s a r e somewhat higher than t h o s e of t h e dry mixtures. T h u s , moisture t e n d s t o d e c r e a s e t h e e x p l o s i v e a r e a by lowering t h e upper limit and i n c r e a s i n g t h e minimum oxygen n e c e s s a r y t o a f f e c t combustion. B e c a u s e t h e d a t a u s e d t o c o n s t r u c t t h e ternary d i a g r a m s were obtained a t c o n s t a n t p r e s s u r e , c h a n g e s in s a t u r a t i o n temperature were n e c e s s a r y t o vary t h e amount of cumene. 1957
T h u s , t h e points of the c u r v e w e r e obtained a t different temperatures. T h e s e temperatures varied from 96" t o 172'F. in t h e dry c a s e and from 9 4 " t o 1 4 2 ° F . in t h e wet case. Temperature and Pressure Rise. In addition t o other d a t a , t h e p r e s s u r e r i s e and t h e temperature r i s e developed during a n e x p l o s i o n of t h e cumene mixtures w e r e measured. B e c a u s e most e x p e r i m e n t s were conducted near t h e l i m i t s , only moderate temperature and p r e s s u r e rises were obs e r v e d . P r e s s u r e r i s e s of 2 t o 2 0 pounds per s q u a r e i n c h gage were common; however, i n one c a s e a p r e s s u r e of 260 pounds per s q u a r e inch g a g e w a s reached f r o m an i n i t i a l p r e s s u r e of 115 pounds per s q u a r e i n c h gage. In most c a s e s , the temperature r i s e w a s from 0 t o 5 ° F . ; although, in t h e c a s e mentioned a b o v e , a 30°F. rise w a s noted. T h e amounts of t h e temperature and p r e s s u r e r i s e are probably related t o t h e s i z e and s h a p e of t h e combustion chamber a n d are not a b s o l u t e v a l u e s for t h e mixture concentration.
PROBABLE ERRORS In addition t o t h e regular errors of mechanical i n a c c u r a c y (pressure and temperature readings), unique errors a s s o c i a t e d with t h i s work include t h e u s e of a c l o s e d s y s t e m , temperature of t h e combustion chamber, and the cumene employed. T h e e x p l o s i o n t e s t s were a l l conducted in a n e n c l o s e d s y s t e m . T h u s , a p r e s s u r e w a s built up within t h e apparat u s during e a c h p o s i t i v e experiment. In t h o s e n e a r limiting c a s e s where p a r t i a l burning would occur, t h e p r e s s u r e built up by t h i s burning may h a v e been sufficient t o c a u s e complete burning; h e n c e , t h e mixture would b e reported explosive. T h i s error may lead t o s l i g h t l y wider limits than would b e obtained with a n open or a no-pressure r i s e system. Another factor affecting t h e explodability of mixtures i s t h e temperature of t h e combustion chamber. B e c a u s e t h i s w a s kept 10°F. h o t t e r t h a n the saturator, t h e t e s t mixture w a s a l w a y s ignited at a temperature somewhat greater t h a n that used t o vaporize t h e liquid. T h i s h e a t may a l s o c a u s e widening of t h e l i m i t s , b e c a u s e some near-limiting mixtures at s a t u r a t i o n temperature burn at t h e e l e v a t e d temperature of t h e combustion chamber. However, in t h e temperature region i n v e s t i g a t e d , t h i s effect a p p e a r s to b e s l i g h t (7, 8). Dow commercial grade c u m e n e w a s u s e d i n making t h e s e determinations. T h i s hydrocarbon a n a l y z e d 93.3% of cumene by freezing point. T h e p r e s e n c e of s l i g h t l y more volatile impurities, s u c h a s e t h y l b e n z e n e , may h a v e lowered t h e upper e x p l o s i v e l i m i t s somewhat, b e c a u s e more hydrocarbon would b e p r e s e n t i n t h e vapor t h a n w a s c a l c u l a t e d for any given temperature. NITROGEN -WATER
EXTRAPOLATED
M O L E PER CENT NITROGEN - WATE
OXYGEN
20
17 5
I5
12 5
IO
M O L E PER C E N T OXYGEN F i g u r e 5.
E x p l o s i v e limits of wet curnene-oxygen-nitrogen
CHEMICAL AND E N G I N E E R I N G D A T A S E R I E S
6
T h e over-all n e t e f f e c t of t h e errors examined in t h e preceding paragraphs i s believed t o b e small a n d on t h e c o n s e r v a t i v e side.
CONCLUSIONS T h e primary purpose of t h e p r e s e n t investigation, t h e definition of s a f e operating c o n d i t i o n s for t h e a i r oxidation of curnene, w a s realized. By t h e u s e of t h e a p p a r a t u s described, i t w a s p o s s i b l e t o find t h e e x p l o s i v e limits of various cumene-air and cumeme-air-water mixtures a t ele v a t e d p r e s s u r e s . An i n c r e a s e in p r e s s u r e c a u s e d a widening of t h e upper limits for both s y s t e m s s t u d i e d . Water c a u s e d a d e c r e a s e i n t h e upper e x p l o s i v e limit. T h e d a t a accumulated a r e of fundamental i n t e r e s t in t h e field of hydrocarbon flammability.
LITERATURE CITED (1) Armstrong, G. P . , B e w l e y . T . , Turck, K. H. W. (to Hercules Powder Co.). U.S. Patent 2 , 6 1 9 , 5 1 0 (Nov. 25, 1952). (2) Armstrong, G. P . , Bewley, T . , Turck, K. H. W. (to Distillers Co., Ltd.), Brit. Patent 653,761 (May 23, 1951). (3) Ibid., 6 9 9 , 0 4 7 (Oct. 28, 1953). (4) Coward, H. F . , J o n e s , G. W., U.S. Bur. Mines, Bull. 503, 1-10 (1952). (5) J o n e s , G. W., U.S. Bur. Mines, Rept. Invest. 3787 (1944). (6) Kennedy, R. E., Spolan, I., Scott, G. S., Ibid., 4812 (1951). ( 7 ) O'Neal, Cleveland, Jr., Natl. Advisory Comm. Aeronaut. Tech. Note 3446, 28 (April 1955). (8) Zabetakis, M. G.,Scott, G. S., J o n e s , G. W., Ind. E n g . Chem. 43, 2120 (1951). Received for review September 4, 1956. 1957. D i v i s i o n 'of Petroleum Chemistry, Atlantic City, N. J . , September 1956.
Accepted March 15, 130th Meeting, ACS,
Sorption of Water Vapor by Thermally Treated Lignite at Dif f e rent Re I at ive H u m id i t ies WAYNE R. KUBE University of North Dakota, Grand Forks, N.D. T h e mechanism of t h e coalification p r o c e s s , by which wood products a r e transformed through t h e various c o a l r a n k s , h a s received attention from numerous i n v e s t i g a t o r s . Generally, it i s believed t h a t t h e woody material a d v a n c e s in rank by a slow chemical p r o c e s s c o n s i s t i n g i n part of dehydration and decarboxylation, occurring over geological ages. O n e a v e n u e of approach t o b a s i c coalification mechanism and an understanding of fundamental c o a l s t r u c t u r e h a s been a study of t h e forms and occurrence of moisture ass o c i a t e d with t h e c o a l s u b s t a n c e . G a u g e r (2) recognized that water w a s recoverable from coal from five s o u r c e s , including: 1. Decomposition of organic m o l e c u l e s 2. 3. 4. 5.
Surface-adsorbed water Capillary-condensed water D i s s o l v e d water Water of hydration of inorganic constituents of coal
Moisture included in t y p e s 2 , 3 , and 4 h a s b e e n s t u d i e d by water vapor sorption-desorption t e s t s a s applied particul a r l y to lower rank f u e l s , which a r e c o n s i d e r e d t o h a v e some g e l l i k e properties. L a v i n e (IO) summarized p r e v i o u s work and presented d a t a on t h e dehydration-hydration of wood and natural lignite. L a r i a n a n d o t h e r s ( 9 ) extended h e i n i t i a l investigation by t e s t i n g p e a t and brown c o a l , and Zromulgated t h e p o s s i b l e c l a s s i f i c a t i o n of North American f u e l s i n t e r n s of pore s i z e a s determined by t h e sorption s t u d i e s . Gordon, L a v i n e , and Harrington (3) s t u d i e d t h e effect of temperature and p r e s s u r e on sorption of water vapor by lignite. Sorption s t u d i e s of t h r e e b a s i c t y p e s of natural lignite-woody, earthy, and peaty-were reported
46
INDUSTRIAL AND ENGINEERING CHEMISTRY
by T a s k e r (15) of t h e Ontario (Canada) R e s e a r c h Foundation for multiple desorption-sorption c y c l e s wherein slightly different sorption c h a r a c t e r i s t i c s were noted for e a c h type. Several i n v e s t i g a t o r s , among them Klein (6) and more recently T e r r e s (16), showed that thermal treatment above t h e temperature required t o i n i t i a t e decarboxylation i n addition t o removal of t h e normally considered moisture r e s u l t e d i n artificial coalification or an a c c e l e r a t e d metamorphism which a d v a n c e s somewhat t h e rank of t h e s o l i d fuel treated. K u b e (8) exposed s a m p l e s of North Dakota l i g n i t e which h a d been thermally t r e a t e d at various temperatures t o 950" F. i n an atmosphere s a t u r a t e d with water vapor at room temperature, and reported t h a t differences i n resorption of water vapor e x i s t e d , depending upon t h e treati n g temperature. Interest i n t h e fundamental properties of North Dakota lignite h a s continued a t a high l e v e l b e c a u s e of t h e l a r g e r e s e r v e s (some 350 billion tons) of t h i s low rank f u e l (1). T h e s e d e p o s i t s represent a major untapped power and chemi c a l s o u r c e in t h e United S t a t e s . T h e present report r e p r e s e n t s a @ion of t h i s continuing interest and e x t e n d s t h e fundamental water vapor sorptiondesorption t e s t s t o thermally t r e a t e d l i g n i t e representing l i g n i t e s originally obtained from a wide a r e a covering t h e North Dakota d e p o s i t and i t s extension into Canada. T h e major o b j e c t i v e s of t h i s investigation were t o determine: 1. The influence of thermal treatment and type of lignite on sorption of water vapor by lignite. 2. The approximate i n c r e a s e in coalification c a u s e d by thermal treatment at temperatures sufficiently high to initiate decarboxylation of the colloidal lignitic substance.
VOL. 2, H 0 . I
