INDUSTRIAL ;IND ENGISEERISG CHE;I.IISTRY
June, 1926
585
Chemical Reactions in the Gas Producer' By John A. Goff DEPARTMENT OF h f E C H A N I C A L
H E d i re c t application of the laws of chemical e q u i l i b r i u m to the problem of the gas producer yields information of interest and of a certain amount of importance. SeumanZ and Husson3 have discussed the e q u i l i b r i a involved from a theoretical standpoint, but the methods employed by them are essentially different from those of the present article which, it is believed. possess certain advantages of simplicity and clearness.
T
Assumptions
It may first be assumed
EXGINBERING,
I t is of interest to know just how much information relative to the chemical reactions taking place in the gas producer can be obtained from the direct application of the laws of chemical equilibrium to the problem. With certain assumptions as to the manner of operation of the producer and the character of the fuel used, the following data can be obtained for any given ratio of steam to air in the incoming blast: (a) the temperature a t which the producer will operate continuously, (bj the composition of the resulting producer gas, and (cj the thermal efficiency of the producer. The ratio of steam to air in the blast which will give maximum thermal efficiency can also be determined. Incidental to the main purpose of the analysis, i t is shown t h a t although five chemical reactions are possible in the formation of producer gas, only three,of these are independent from the standpoint of chemical equilibrium, and as far as the final results are concerned, only three need be considered. Choosing three convenient to our purpose, i t is then possible to determine the amount of fuel burned according to each of these three reactions for any given ratio of steam to air in the blast.
that the producer gas will contain no free oxygen. The experiments of Haslam, Entwistle, and Gladding' show that when a mixture of air and water vapor enters a gas producer the free oxygen in this mixture is completely consumed in the first 3 or 4 inches of the fuel bed above the ashes, and that carbon dioxide alone is formed according to the reaction
c
+
0 2
=
r N I V E R S I T Y OF ILLISOIS, U R B A N A , ILL.
coz
(A)
Furthermore, it may be assumed that, together with Reaction A, only four more Are possible-namely, = Hs + CO + + HDO 2H20 = 2H1 + C02 c + COa = 2 c o CO + Hz0 = C02 + H2
CO = x mols con = y Hz = z
HzO = umols ND v
There are certain relations between these quantities, which state that the total number of molecules of X2, H z J and 1 2
4
Received October 22, 1928. Sfahl u. Eisen, 33, 394 (1913). Rer. i n d . minerale, 1, 373 (1922). THISJ O U R N A L , 17, 886 (1925).
= 3.8 u + z = u
2'
1,'*x
+ 3: +
=
1,'DU
1
+
1/2u
and the above schedule becomes
co
= x = y 24' - 2 Hz = x u - (X 2y H?O Nz = 3.8
cos
+
+
- 2)
The amount of water vapor, u, introduced with the air
can be fixed quite arbitrarily, but if too much is introduced the temperature of the gas p r o d u c e r will steadily decrease, while if too little, it will rise. If it be stipulated that just enough water vapor, CT,be introduced with the air to maintain a constant temperature T , then u can be expressed linearly in terms of z and y, the coefficients being functions of the temperature. This is possible because, no matter what final gas mixture is assumed, i t can be thought of as the result of three reactions. That is, suppose that 1 mol of carbon is oxidized by Reaction A, the result is
co = o co*= 1
C C
I n using these five reaction equations i t is assumed that the fuel fired to the gas producer is pure carbon free from hydrocarbons such as methane, or a t least that not any of these hydrocarbons exist as such in the final gas mixture. Again, it is assumed that the final gas mixture is in chemical equilibrium with the carbon in a certain zone in which the temperature can be regarded as uniform. Specifically, let it be supposed that surrounding the fuel bed of carbon a t the temperature T (" F. abs.) there is a certain mixture of gases also a t the temperature T. According to the previous assumptions this gas mixture will contain only CO, Cog, Hz, HzO, and Nz, the free oxygen in the air introduced having disappeared completely. Hence the following schedule can be written:
O2 are constant. Supposing that the mixture of air and w a t e r v a p o r entering the producer contains 4.8 mols of air plus u mols of water vapor, the desired relations are
HzO N2
u
3.8
HZ = 0
Then let z mols of carbon be reduced by Reaction B, changing the gas composition to
co coz H2
Hz0 u - x Nz = 3.8
= Y
= 1 x
and finally let (y C, thus making
- 1) mols of carbon be reduced by Reaction
co = x coz= y
Ha = ~ + 2 ~ - 2 (X 2y - 2) HzO= u Nz 3.8
-
+
which is the general composition. The heats of reaction of the Reactions 8,B, and C can be found from data given by Goodenough and Felbeck,6 as follows:
+ +
+ 174,290 B. t.B.U. t. U. ++COCOa- -49,460 31,370 B. t. U.
C 0 2 = COz C HrO Hz C t 21320 = 2H2
(A)
(B) (C)
To be sure these quantities depend on the temperature, but that variation is small and can be neglected. The total heat evolved in forming the gas mixture to be studied is then 5
University of Illinois Expt. Sta., Bull. 139.
INDUSTRIAL Ai\-D ENGINEERING CHEMISTRY
586
Qd = 174,290 - 49,460 x - 31,370 (y -
1)
Now this heat is assumed to be used up in three ways: ( a ) to heat Q mols of water a t temperature t = 62’ F. to the boiling point corresponding to atmospheric pressure, vaporize it, and
Vol. 18, No. 6
dx, mols of carbon be gasified by Reaction D dx. mols of CO enter into Reaction E
Then the number of mols of H2formed will be dnH, =
dXb
+ 2dxc + dx,
and similarly
- dxd + dx, - d X b - 2dX, - dx,
dnc02 = dx, dnH,O =
dnb = dnco =
- dxb - dx, - dxd d X b + 2dxd - dx,
Kow it can be shown that a criterion for chemical equilibrium in a mixture of gases is the vanishing of the sum ZfSzdn,. In this sum the dn’s have the meaning already assigned to them, and the f’s are certain functions of the temperature and the partial pressures. If this sum be formed and the terms rearranged, the following expression results: = +- 2fHi0 ++ dQ(2fc0 dxe(2f~2 -fc) - fc) -k d d f m - fco + fcoa -
zfd%
dXb(fHt
fEi0
f C 0
fC)
+fCOz
~ C O Z
~HZO)
orZf,dn, = Cldxb
+ Czdxc + Cadxd+ C4dx.
But in order to have stable equilibrium the above expression must vanish for all possible values of the d Y s and hence each coefficient must vanish separately. However, these coefficients are connected by such relations as raise the temperature of the vapor formed to the temperature T; from data given by G o ~ d e n o u g h this ,~~~ amount of heat is found to be given by the expression Q. = ~ ( 1 3 , 8 3 0 9.152 T ) ( 6 ) to heat 4.8 mols of air from t = 62’ F. to the temperature T , the heat required being given by Che expression Q b = 4.8 X 7.221 X ( T - 522) Q b = 34.66 T - 18,090
+
and ( c ) to supply radiation and conduction losses Qr. This quantity of heat would perhaps be proportional to the temperature under which the gas producer is operated, but for the present discussion will be taken to be zero although to do otherwise would not complicate the problem. i).heat balance will be established if Q e = QG f Q b Qr or, with Qr = 0, if
Cf CI
- c1 = c,
c 1 - c a
= 2c4 = c 4
so that, instead of four independent equilibrium conditions, there are but two, for making any two of the coefficients vanish insures the vanishing of the other two. The equilibrium equations most convenient for our purposes are the following :
+-
2fCO fH2
-f c = 0 -f C 0 -fHz0
(1)
fCO2
fCol
= 0
(2)
The function f is defined by the equation f = i - Ts
+
13,830
o
- 1) = +( y 9.1520 T + 34.66 T - 18,090
an equation which expresses
u
linearly as follows:
174,290
- 49,460 x - 31,370
L
-
-
mx ny 223,750 - 34.66 T L= 13,830 9 . 1 m 49,460 m = 9.152 T 13,830 31,370 n = 13,830 9.152 T
u =
+ + +
Derivation of Equilibrium E q u a t i o n s
\Ye wish now to make use of the assumption that the final gas mixture shall be in chemical equilibrium. For this purpose suppose that the mixture should suffer a change due to one or all of the Reactions B, C, D, E. I n particular, let dxb mols of carbon be gasified by Reaction B dx, mols of carbon be gasified by Reaction C e ‘Properties of Steam and Ammonia.”
+
where i is the thermal potential (u pv) of the gas in B. t. u. per mol, T is the absolute temperature in degrees Fahrenheit, and s the entropy per mol which is expressible in terms of the specific heat, y, of the gas a t constant pressure arid the partial pressure, p , of the gas as follows: s =
j $dT + so - R log, p
INDUSTRIAL AND ENGINEERING CHEMISTRY
June, 1926
so and R being constants. When the expressions for the various f's are substituted in (1) and ( 2 ) these lead to the ordinary forms of the equilibrium equations-namely,
Table 11-Values Temp. F. Abs.
L m
n
of the Coefficients L rn, and R as Computed from Equations Alread; Deduced
1300 1500 1600 1800 2000 2100 2400 6.9447 6.2321 5.9103 5.3254 4.8060 4.5676 3.9272 1.9223 1.7946 1.7370 1.6323 1.5390 1.4965 1.3815 1.2192 1.1382 1.1017 1.0353 0.9763 0.9492 0.8764
Table 111-Values Temp. F. Abs.
wherein A , and A, are functions of the temperature T, which can be computed from data given by Goodenough and Felbeck5,7 the values of which are tabulated in Table I. Table I-Values Temp. F. abs. Log A c Log A ,
of Log Ac and L O 8 A, Computed from Previous Data591
XO
YO
co
COz
1300 1500 1600 1800 2000 2100 2400 3.7295 2 3056 2.9433 Q.0023 0.8414 1.1997 2.0849 1.0108 1 4007 1.5552 1.8074 0.0018 0.0828 0.2778
of
xo
Hz Hz0 N2
and Y O Which Satisfy the Equilibrium Equations
1300 1500 1600 1800 2000 2100 2400 0.0758 0.3744 0.8772 2.0022 2.5180 2.5569 2.4795 0.9840 1.0027 0.9686 0.5162 0.1260 0.0582 0.0075
Table IV-Gas Temp. ' F. Abs.
587
1300 0.72 9.37 0.42 53.19 36.30
Compositions in Per cent by Volume 1500 3.91 10.45 3.96 42.05 39.63
1600
9.78 10.84 9.08 27.95 42.35
1800 25.50 6.58 13.20 6.23 48.49
2000 34.70 1.73 10.60 0.52 52.45
2100 36.00 0.82 9.47 0.18 53.53
2400 36.55 0.09 7.29
O.OO+
56.07 ------100
100
100
100
100
100
100
Table V-Miscellaneous Calculations F. Abs. 1300 1600 1600 1800 2000 2100. 2400 Water V a a o r Inlroduced inlo Gas Producer wilh Air Mols steam per 4.8 mols air ( u ) 5.599 4.419 3.320 1.523 0.808 q.686 0.495 Temp.,
Lbs. steam per Ib. air
(0.13~) 0.728 0.575 0.432 0.198 0.105 0.089 0.064 Lbs. steam per lb. carbon consumed 7.820 4.670 2.700 0.908 0.459 0.395 0.298 Gas Heating Value at 1 = 6 Z a F. 1312 8820 21,256 44,640 53,160 53,545 51,968 B. t. u. per mol. 3.45 23.2 55.8 117.0 139.5 140.5 136.5 B. t. u. per cu. f t . Eficiency of Gas Producer" 7.43 35.48 59.24 79.75 83.66 83.54 81.32 Per cent Heating value in B. t. u. per mol 'Heating value of carbon in producing 1 mol producer gas
Determination of Amount of Fuel Burned Absohfe &mperafura
- +.
/I
#
Each of the partial pressures is expressible in terms of x and y, since the composition of the gas mixture is now
co = x co2 = y
.
+ --2n y - ( x + 2 y - 2 )
Ht = x 2y H 2 0 = L - mx s 2 = 3.8 Total = L mx
-
c+
C C
- n y + x + y + 3.5
The usual procedure of suppressing the partial pressure of the carbon has been followed which amounts to assuming that the solid carbon exerts the total pressure, P. Assuming further that the total pressure P is one atmosphere ( P = l),the equilibrium equations can be written X2
y (L - w x
- ny
+
It was shown that any given possible gas composition consistent with our original assumptions can be looked upon as the result of Reactions A, B, and C taken together, 1 mol of x mols entering B, and (y - 1) mols entercarbon entering -4, ing C. However, with one exception all the values of y found make (y - 1) negative, indicating that Reaction C would have to proceed from right to left. That is, if the three reactions
+ 3.8 + x + y )
y (X 2Y - 2) x(L-mx-ny-x-2y+2)
=
=
A, A,
co2 + HzO = CO + Hz + 2Ha0 = COz + 2H2 0 2
=
are assumed to be the only reactions involved, which is entirely possible, then the last one listed must proceed from right to left with the formation of water vapor, instead of from left to right with the formation of carbon dioxide. A somewhat better combination consists of the three reactions taken in the following manner: 1 mol of carbon
-
(1)
enters Reaction A;
(2)
action C; and x/2 mols Reaction D - C COS = 2CO. The resulting gas composition is the same as before, so that this combination is just as good as the preceding and has the
and these equations are to be solved simultaneously for values of x and y. The method used is to choose a particular temperature T = TOand compute the constants L, m, n, A,, and A,; then by trial to find a value of y = yo which yields the same value of zo from either of Equations 1 and 2 . Having found zoand YO,the gas composition in mols can be written down and then reduced to a percentage composition. Also u can be computed. Other related data are presented in Tables I1 to V and Figures I and 11. 'See also Lewis and Randall, J . Am. Chcm. Soc.. 37, 464 (1915).
+
mols of carbon enter Re-
+
advantage that none of the quantities 1,
-t- 22y
-
2, end
z/2 is negative for any temperature considered. The ratio X+Y
will, in this case, represent the fraction of the total
carbon which is burned according to Reaction A,
x+Zy-2 2(x y) +
that according to Reaction C, and
X
2(x
+ Y)
that according
to D. These results are tabulated in Table VI and exhibited graphically in Figure 111.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
588 Table VI-Distribution
of Total Carbon among Three Principal Reactions
Temp., a F. Abs. 1300 REACTION A C 02 = C o r 94.3 C C 2Hz0 = COz f 2H2 2.1 D C COz = 2CO 3 . 6
++ +
1600
1800
2000
2100
2400
72.6
54.2
39.7
37.8
38.3
40.3
13.6 13.8
22.1 23.7
20.5 39.8
14.6 47.6
12.9 48.8
9.9 49.8
1500
- - - __ __ __ 100
100
100
100
100
100
100
Vol. 18, No. 6
5-The efficiency of the producer increases with the temperature a t which it is operated up to 2000' F. and then decreases because the amount of heat carried away by the hot gases leaving the producer increases with the temperature. 6-The amount of steam in the air blast required to maintain a given temperature is less the higher the temperature. 7-For maximum efficiency, under the assumptions made, approximately 0.4 pound of steam per pound of carbon should be used (Figure IV).
Conclusions
I n discussing the results of the present analysis, it is well to Testate the assumptions under which it is made: 1-The producer gas contains no free oxygen, it having entirely disappeared according to Reaction A, which is complete at all temperatures considered. 2-The fuel fired t o the producer, which is operated continuously, is pure carbon. 3-The producer gas is in chemical equilibrium with the hot carbon a t the temperature T and under atmospheric pressure. 4-Just enough steam is introduced with the air to maintain a constant temperature. 5-Radiation and conduction losses have been disregarded.
Under these assumptions, me can conclude that: 1-The nitrogen content of the producer gas increases with the temperature because less and less steam is being introduced with the air blast. 2-Above 2100" F. the carbon dioxide and water practically disaaaear, a statement which has been made bv F. Haber.8 3-If operated a t a temperature of 1750"-F. a maximum hydrogen content is obtained, but a t higher temperatures the hydrogen content drops while the carbon monoxide content rises and remains practically constant. 4-At very low temperatures (1300' F.), the water or steam goes through the producer practically as such and enters but slightly into reduction reactions. 8
"Thermodynamics of Technical Gas Reactions," p. 313.
I
LA S h m Ukf per Lb. Carbon
8-Out of the five possible reactions (A, B, C, D, E) it is only necessary to consider three principal reactions, of which one must be Reaction A but of which the other two may be chosen at will. If Reactions A , C, and D are taken to be the principal ones, Figure I11 shows that at low temperatures A is most effective, while at high temperatures C plays a minor part, and -4 and D go together to form carbon monoxide.
Piperonal in Vanilla Extract' By C. B. G n a d i n g e r MCLAUGHLIN-GORMLBY-KING Co., MINNEAPOLIS, MI".
HE use of piperonal (heliotropine) in vanilla extract
T
is largely due t o the belief that this substance is a natural constituent of vanilla beans. Krebs2 called .attention to the heliotrope odor of Tahiti beans, while Busse3 and Go11er4 suspected that both vanillons and Tahiti beans contained piperonal. These opinions, which were based on the odor of the beans, have been given prominence in literature, but it has not been proved that piperonal occurs in any variety of vanilla beans. Wahlbaum5 concluded that Tahiti vanilla does not contain piperonal. Little attention has been paid to the use of piperonal as a n adulterant of vanilla extract, and it seemed worth while to determine if extracts of known purity made from the different varieties of beans respond to qualitative tests for this compound. The results obtained made it desirable to investigate the composition of vanillons. Vanillons are said to be obtained from the vines of V . pompona, V . guianensis, and undetermined species.3 They cannot legally be used in vanilla extract. Received January 20, 1926. Pharm. Zenfralhalle, 36, 487 (1895). Arb. kais. Gesundh., 16, 107 (1898). 4 Pharm. Zenfralhalle, 46, 192 (1904). 5Schimmel & Co., Semiannual Report, October, 1909, p. 142. 1 2
a
Experimental
Extracts of standard strength were prepared from Mexican, Bourbon, South American, Java, and Tahiti beans and from vanillons; the beans were obtained from different sources. These extracts were subjected t o the phloroglucinol test and the gallic acid test for piperonal. The phloroglucinol test was applied in this manner: Fifty cubic centimeters of extract were de-alcoholized by evaporating spontaneously before a fan t o about 40 cc. and transferred to a separatory funnel with water. The solution was extracted once with 50 cc. of ether, and the ether solution was separated, washed three times with 15-cc. portions of 2 per cent sodium hydroxide solution, and once with 15 cc. of water. The ether was then evaporated spontaneously, just t o dryness, in a porcelain dish. A few minute crystals of phloroglucinol were added, followed by a few drops of concentrated hydrochloric acid. A deep red color, similar t o that produced by vanillin, is formed if piperonal is present.
The Mexican, Bourbon, South American, and Java extracts gave negative results; the Tahiti extracts gave doubtful reactions; and the extracts of vanillons yielded strong positive reactions. Bourbon extract containing 0.005 gram of added piperonal per 100 cc. yielded a strong red color. The first four varieties of beans named apparently do not
