‘The increase i n nickel activity with pressure seemed related to a n increase i n the surface covered by the adsorbed CO. T h e temperature also had a significant effect on the reactivity of the nickel surface during the reaction. although the cause of this effect \vas not determined.
0
= = u, = u,“ = x =
u
time, min. gas velocity, cm.,isec. weight of nickel present a t time 0. mg. initial weight of nickel in charge, mg. conversion o r fraction of nickel charge mg. nickel reacted reacted, mg. nickel charged
Nomenclature = activity of nickel, mass of nickel i n reactive state total mass of nickel present a’ = initial activity o r activity a t time 0 = 0 = specific heat, cal./gram-o C. CP 1G; = free energy change a t temperature T . kcz.1.‘gram-mole AH;-namic driving force in terms of carbon monoxide partial pressure difference, atm. = total pressure of system. atm. Pi. r = specilic reaction rate, rnass of nickel reacted miri.-mass of nickel present = standard entropy of formation a t 298” K., S,, cal. /gram-mole-o K. = temperature, O C. = fracrion of surface covered by adsorbed gas
literature Cited
d:
(1) Garratt, A. P., Thompson, H . \V., J . Chem. SOC.1934, 1817. (2) Goldberger, W. M., doctoral thesis in chemical engineering, Polytechnic Institute of Brooklyn, 1961. (3) Hougen, 0. A., Watson, K. M., “Chemical Process Principles,” Pt. 3, LViley, New York, 1947. (4) Kelley, K . K., “Entropies of Inorganic Substances,” Bur. Mines Bull. 477 (1948). (5) Kelley, K . K . , “High Temperature Heat Capacity and Entropy Data for Inorganic Substances,” Bur. Mines Bull. 476 ( 1949). (6) K:bachewski, O., Evans, E., “h.ietallurgica1 Thermochemistry, 3rd ed., Pergamon Press, New York, 1958. (71 Mittasch. A , . Z. Phvsik. Chem. 40. 1 (1902) (8) Othmer, b. F.,Znd.’Eng. Chem. 36, 6h9 (1944). (9) Pospkhov, D. A , , Zhur. Obshche‘l Khim. 18, 610 (1948). (10) Rea, A. E., “Bibliogiaphy on Nickel Carbonyl,” International Nickel Co.. New York. 1955. (11) Spice, J. E., Stavely, L. A. K., Harrow, G. A , , J . Chem. Soc. 1955. 100. (12j S;inivasan, V., Krishnaswami, K. R., Current Sci. (India) 2 5 , 328 (1956). (13) Sykes, K. LV., Townshend, S. C., J. Chem. SOC. 1955, 2528. (14) Trapnell. B. M. IV., “Chemisorption.” Butterworth’s, ‘ Londok, 1955. (15) Trautz, M., 2. rlnorg. Allgem. Chem. 104, 169 (1918). (16) \\’ikon. A . J. C., J . Inst. ‘Met. 70, 543 (1944). \
I
~~~
I t >
RECEIVED for review December 26, 1961 ACCEPTED NoLember 23, 1962
e,
H E A T OF R E A C T I O N OF A I R BLOWING A S P H A L T DOUGLAS B. S M I T H AND H E R B E R T E . S C H W E Y E R Unioersity of Florida, Gainesaille. Flu.
This research was undertaken for the purpose of measuring the exothermic heat of reaction of air blowing asphalt. The differential heat of reaction, defined as the quantity of heat liberated’per pound of asphalt per degree Fahrenheit change in softening point, was measured by making a complete heat balance on an air blowing reactor. It was found that the differential heat of reaction decreased with increase in softening point for all four asphalts studied. The integral or total heat of reaction in air blowing asphalts to a softening of 250‘ F. was found to b e about 85 6.t.u. per pound for two of the asphalts studied and 260 6.t.u. per pound for the other two. A procedure for calculating the heat duty of industrial air blowing reactors is illustrated using the differential heat of reaction values and kinetics data.
ALTHOUGH
!T IS WELL KNOM-s that the asphalt air bloiving reaction is exothermic, no information was found in the literature concerning the magnitude of the heat of reaction. ’This information \voulcl be of considerable practical value. since it is necessary in the operation of industrial air blowing reactors to provide a method of removing rhe excess heat generated by the reaction. This has been done by spraying water on the outside of the reactor vessel and cutting back the air f l o ~ vrate to s l o ~ vdown the reaction Fvhen the reaction mixture becomes too hot. A knowledge of the magnitude of the heat of reaction a n d reaction kinetics would enable the designer to calculate the heat duty a n d design a n adequate cooling
system for the reactor, thus permitting continued operation a t a uniform air flow rate. A procedure for measuring the heat of reaction has been developed by Smith ( J ) . TO measure the heat of reaction. it is necessary to select the change in some physical property of the charge as a measure of the degree of completion of the reaction. because the pure chemical kinetics of the reaction are not knobvn. There are many complex reactions occurring, a n d \vork in this laboratory is now being directed toivard a more sophisticated measure of the degree of completion of the reaction. For the present \vork: the change in ring a n d ball softening point was selected as a measure of reaction r a t e ; it has cerVOL. 2
NO. 3
JULY
1963
209
AIR
I
GAS PREHEATER
4 c 3
c U
(r
L HEATING ELEMENTS
PRESSURE GAOE
5
c Q J
E a DISTRIBUTION
THERMOMETER
f
SAMPLE O U T L E T
REACTOR VESSEL
Figure 1.
REGULATOR
I'
0
300
POWER SUPPLY
Schematic diagram of apparatus
tain definite advantages for the practical applications of these results. The amount of heat liberated by the reaction per pound of charge over a n incremental change in softening point can be called the differential heat of reaction over that softening point range. This differential heat of reaction, in B.t.u. per pound per degree softening point, can be measured over several softening point ranges. When these values are plotted against softening p i n t , the area under the curve will be the integral heat or reaction. Experimental
Design of Equipment. The experimental asphalt blowing apparatus (Figure 1) was designed to blow about 5 gallons of asphalt in a batch process. The reactor was of stainless steel construction, 14 inches in diameter and 20 inches in height. \Vhen charged it was about half full of asphalt, with the remaining volume available for vapor. A gas disperser was installed in the bottom of the reactor to ensure complete mixing. An outlet line was installed in the bottom of the reactor for taking samples and draining the reactor at the completion of the run. The reactor was charged through a 3.5-inch-diameter opening at the top which was capped during operation. Exit gases left through a 0.75-inch outlet line. To maintain approximately isothermal operation, it was necessary to use 6 inches of insulation on the reactor when blowing the asphalts with low heats of reaction and 3 inches of insulation when blowing the asphalts with higher heats of reaction. An air preheater, designed to elevate the temperature of the incoming air to the temperature of the reactor, consisted of a steel box containing six 1000-watt conical heaters. These heaters were wired in series on two 220-volt circuits, three heaters on each circuit. The only control on one circuit was off-on, but the other was wired through a voltage regulator so that the voltage through this circuit could be controlled over the entire range of 0 to 220 volts. Temperature control of the air entering the reactor was maintained by this voltage control of the preheater heating elements. Before entering the preheater, the inlet gas was metered by a flowmeter. The inlet gas was either air or nitrogen. The air was used as the reactant and the nitrogen was used to mix the asphalt prior to initiating the reaction with air and to obtain cooling curves by which the heat loss of the system was evaluated. By means of valves it was pqssible to switch immediately from air operation to nitrogen operation. This became necessary when the temperature of the system became too high. \Yith nitrogen flowing through the reactor the temperature dropped to a point at which the reaction was reinitiated. The temperature of the reacting asphalt was not allowed to exceed 535' F. during these studies. Operating Procedure. A weighed quantity of asphalt (43 to 50 pounds) was charged to the reactor and the charge was heated with immersion heatrrs. After the charge had reached the proper temperature, about 525" F., the heaters were removed, and the reactor was capped. Nitrogen, preheated to the reactor temperature, was then introduced to the reactor to mix the asphalt in the reactor completely and ensure a uniform asphalt temperature throughout the reactor. Temperatures of both the incoming nitrogen and the asphalt charge were reaa every 5 minutes, with the nitrogen temperature being regulated to the same temperature as the asphalt. 210
200
I00
I & E C PROCESS D E S I G N A N D DEVELOPMENT
500
400
600
700
800
TIME, MINUTES
Figure 2.
Time-temperature profile, T = f(t)
When thetemperatureof the asphalt had dropped to about 500" F. the incoming nitrogen was replaced by air which reacted with the asphalt. As the reaction proceeded, the asphalt temperature rose because of the exothermic heat of reaction. The temperature of the air was increased to maintain it at the asphalt temperature. During this period, readings were made every 5 minutes of the asphalt temperature, incoming air temperature, ambient air temperature, flowmeter position, and system pressure. Samples were taken every 20 to 30 minutes. \$'hen the temperature of the asphalt approached 530' F., the air was replaced by nitrogen and the asphalt temperature dropped again because of normal heat losses of the system. When the asphalt temperature again approached 500' F., air was reintroduced, the reaction was reinitiated, and the temperature of the charge again rose. This procedure was repeated until the rate of the reaction had decreased to the point at which the exothermic heat of reaction was no longer sufficient to maintain the asphalt at reaction temperature. A time-temperature profile from a typical run is illustrated in Figure 2. Section I of the graph shows the period in which the asphalt cools from the temperature to which it was heated to the temperature at which air was first introduced, Section I1 shows the alternate periods of reacting and cooling, and Section 111 shows the period during which the heat of reaction was nearly the same magnitude as the heat losses and was barely sufficient or insufficient to elevate the temperature of the system. The decrease in the rate of temperature rise is rather abrupt. Each sample was measured for ring and ball softening point by the method described in ASTM Test D 36-26 (7). Asphalt which had been subjected to nitrogen blowing showed no change in softening point, indicating that nitrogen does not react with the charge. Four different asphalts were used in these studies. Some of the properties of these asphalts are listed in Table I. Calculation Procedure
T h e differential heat of reaction over a n incremental softening point range was determined by making a heat balance on the system : Input - Output
=
A,,
Accumulation
+ L ) A m + A , f At + A , f A , - ( H a - H,) + L
(Ha iQ ) - (Ho
Q
=
=
(1)
(2)
T h e input and output rerms of the heat balance were evaluated in the following manner :
Ha
=
Fara(T, - Td)At
H , = FoCo(To- Td) 4t
(3) (4)
Since the exact composition of the exit gas was unknown, the value of the heat capacity of the exit gas stream could not be determined. T h e assumption was made that:
F,C,
=
sac,
(5)
I t was assumed that thermal equilibrium was approached between the air and the asphalt during the reaction period,
Table 1. S-310
Description
Gulf Coast residuum
Density, grams/ml. 60" F. 0.967 0.963 77" F. Kinematic viscosity, centistokes 210' F. 289 350" F. 18.2 Penetration, 77' F. 100 gram$/5 sec. Too soft Component analysis0 16.7 Asphaltics, % 64.8 Saturates. % 18.2 Cyclics, % a Determined by method (3j Trader. and Schweyer (5).
Asphalt Properties
South Texas heavy asphalt base residuum 0.991 0.979
A,,
M&,,(Tz
-
Ti)
A , = M,C,( Tp - T I )
I n Lvriting Equation 8, it \vas assumed that the thermal conductivity of the stainless steel reactor and the heat transfer coefficient between asphalt and reactor were infinite. causing the temperature of the entire vessel to be the same as that of the reacting asphalt. This assumption could have a greater effect, perhaps, than any other simp1if)ing procedure, since the film transfer coefficient is known to b'e rather low. If it Lvere as Io\v as 10 I3 t.u./(hr.-' F.-sq. ft.) the magnitude of its resistance Ivould be small for a n over-a 1 heat transfer coefficient of about 0.2, ivhich was calculated for the insulated reactor in these runs. However, in unsteady state operation a film coefficient of 10 would result in a considerable lag in heat accumulating in the reactor and insulation. I n extreme cases, which would occur when the rate of temperature rise is rather rapid, a n error as high as 2570 could occur in the terms A , and A i , or an overall error of 10% in the magnitude of Q . Because of this assumption, the 4H values for low softening point values of S-118 and S-310 could be as much as 10% too high.
43.0 38.9 18.2
related by setting u p a n unsteady state heat balance which yields a second-order partial differential equation. This is given in cylindrical coordinates by:
ET
C,P d T = k dt dr4
+
1T 1 + E3.9 T r 2 de2
(10)
+
r dr
If the assumption is made that heat transfer takes place only dT dT in the radial direction, that is = 0 and 5 = 0, then Equation 10 simplifies to: r d- 2+T- = d- T
dr2
(7)
(8)
227
37.6 37.1 25.3
(6)
I n \+-riting Equation 7, complete mixing was assumed.
1633 54.4
195
6.9 46.1 47.1
By maintaining the inlet air temperature close to the temperature of the asphalt, H a - H, can be made small relative to the othe; terms in the heat balance; thus, any error in this term resulting from the above assumptions uould affect the magnitude of Q very slightly. T h e accumulation terms are evaluated by the following procedure :
1.022 1,016
1345 43.6
Too soft
perature of the asphalt. I n a thoroughly mixed system this assumption is a valid one. T h e n :
S-177 East Texas asphalt base residuum
East Central Texas residuum 1.020 1.017
153 8.1
so the temperature of the exit gas was the same as the tem-
Ha - H
