INDUSTRIAL AND ENGIXEERING CHEMISTRY
April, 1929
The presence of an excess of urea or calcium nitrate increases its hygroscopicity, and when calcium nitrate is present in excess the hygroscopicity of the mixture is greater than that of calcium nitrate alone. Similarly, ammonium nitrate and ammonium sulfate combine to form the compound (NH&S04.2NH4XO3. Such a mixture, containing ammonium nitrate and ammonium sulfate, is shown in the table to be less hygroscopic than ammonium nitrate and the compound itself is undoubtedly even less hygroscopic. It will be observed that Table I1 contains no mixtures of salts which do not form common ions in solution. An example of such a pair of salts is potassium chloride and ammonium nitrate. Such a combination of salts is capable of undergoing double decomposition, which may be expressed for the given example by the equation: KCl
+ NHaNO,
= KN03
+ NHaCl
Since either pair of salts is formed from the other pair, they are known as reciprocal salt pairs. A system composed of
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reciprocal salt pairs and water is one of four components, and there is one and only one temperature and vapor pressure a t which all four salts may exist in solid phases in contact with the solution and vapor phase. Above this particular temperature one salt pair is stable in contact with the solution and below i t the reciprocal salt pair. At the higher temperatures there will be two solubility curves, the solid phases being the stable salt pair and either of the other two salts; a t the lower temperatures there will be likevise two solubility curves, the solid phases here being the stable reciprocal salt pair and either of the salts of the salt pair stable a t the higher temperatures. At any given temperature, therefore, there may be two vapor pressures, each dependent upon the third solid phase present. For this reason it is necessary to know which salt forms the third solid phase a t the temperature a t which the vapor pressure is measured. Vapor pressure measurements on systems involving reciprocal salt pairs are now being made and will be reported in a subsequent paper.
Fuel Economy in the Rotary Kiln Burning Portland Cement Clinker’ Robert D. Pike PIKEA N D WEST, 4068 HOLDENST.,EKERYVILLE, CALIF.
N TWO previous papers2b3 the writer has developed ideas and data which in this paper are employed for calculating rational performance curves of rotary kilns and for predicting the outcome of proposed methods for increasing fuel economy. The calculations involved in this work are rather complex and exceedingly tedious, and no attempt will be made here to explain them in detail. Suffice it to say that the rotary kiln has been studied as a heat-transfer tube made up of three major zones-the clinkering, the calcining, and the1 preheating zones. Each solution or point on any of the curves presented in this paper has involved the fitting together of these three zones by trial and error, to give a self-consistent whole or unit corresponding to the actual rotary kiln. Of the three zones it has been considered that the calcining zone is the most important in the thermal sense, and the laws of heat transfer in this zone n-ere developed analytically, leading to two equations, which furnished the starting point for all the solutions. Equation 1 shows the length of the calcining zone that will give maximum thermal efficiency. If there were no heat losses through the shell to the outside atmosphere, LBmax would be infinite, but by introducing into the analysis a term to allow for these losses Ltmnxbecomes finite. For maximum efficiency
I
Equation 2 gives the general relations for thermal efficiency in the calcining zone:
* Received 2
August 23, 1928.
Pike, IND. ENG. CHEM.,20, 1155 (1928).
S l b i d . , 21, 230
(1929).
= internal diameter of calcining zone of kiln, feet = length of calcining zone of kiln, feet = pounds of gas per minute per square of diameter (mass
velocity)
= temperature of gas entering calcining zone, =
= = =
=
=
= = =
O
F.
temperature of calcination, O F. temperature of outside atmosphere, O F. mean specific heat of gases coefficient of heat transfer to outside atmosphere by conduction and convection, including numerical relation between surface of charge exposed and diameter d, B. t. u. per square foot per minute per O F. coefficient of heat transfer to charge by conduction and convection, including numerical relation between the surface of the charge exposed and the diameter d , B. t. u. per minute per square foot per F. thermal efficiency in calcining zone thermal efficiency when gases leave calcining zone a t temperature T, base of natural logarithms length, in feet, of calcining zone when efficiency = Emax
It has been necessary t o assume, for the sake of simplicity, that the heat transfer numbers K4 and K j are constants for any given mass velocity, K. The effect of mass velocity on these constants has been shown in a previous paper. I n utilizing the above equations as a starting point for the solutions, curves are plotted of which Figure 1 is typical. Equation 1 brings out the vital point that for maximum efficiency there is a definite linear relation between the length and diameter of the calcining zone; and the same is found t o be true of the kiln as a whole. Before proceeding with the presentation of the curves of the complete rotary, the writer wishes to make it clear that these curves are theoretical and not actual. To accumulate data for directly plotting such curves would take a lifetime and would probably not be possible of accomplishment. The phenomena in question are so complex that these curves necessarily have implicit in their make-up certain assump-
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Vol. 21, No. 4
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tions and approximations which make their absolute accuracy questionable. Nevertheless, it is believed that they present a correct approximation toward rationalization of the thermal relations of a rotary kiln, and as such should stimulate research and development along correct lines. This belief is strengthened by the fact that wherever the writer has tested the curves to predict the known results of practice the checks have been fairly satisfactory. Effect of Size of Kiln
Figure 2 shows the relation between length and fuel consumption per barrel for kilns having internal diameters of 6 and 8.5 feet, respectively. They show the gradual diminution of rate of reduction in fuel consumption for progressively greater lengths, and bring out the important point that, for any given length, the 6-foot kiln uses less fuel per barrel than the 8.5-foot kiln, for same rate of firing. This difference becomes less as the length becomes greater, and presumably for kilns of infinite length the fuel consumption would be the same for all diameters. These curves show that for practical lengths the ratio of length to internal diameter has an important bearing on the fuel consumption. The rate-of-firing constant, K1,is that in effect, a t least approximately, in the observed kilns. It should be emphasized that these curves are based upon the assumption that the fuel fired in unit time remains constant.
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Figure 3 shows output us. fuel consumption for four different sizes of kilns. They may be termed ''load characteristic" curves. All the kilns shown in Figure 3 are evidently overloaded in the thermal sense, but shorter kilns are overloaded far more than longer kilns. There is a striking difference between curve 1, for a kiln 125 feet long, and curve 3, for a kiln 225 feet long, in respect to the rate of increase of fuel consumption with increase of output. The curve for the shorter kiln is relatively steep and that for the longer kiln is relatively flat. This distinction is a very important one in favor of long kilns because it means that their output can be forced without increasing fuel consumption to the same extent as in shorter kilns. If one should attempt to verify curve 1 of Figure 3 by an actual test, a certain practical difficulty would be encountered which can best be understood by reference to the work of Grjimailo.4 Probably in no type of furnace does the conception of hydraulics have more significance than in a rotary kiln. It must be obvious that the design should be so carried out that the entire perimeter is wetted by the hot gases. This is accomplished by giving the proper diameter to the upper end of the kiln. For example, in the kiln shown in curve 1, Figure 3, the proper upper diameter, as calculated after Grjimailo for an output of 500 barrels daily, is 47 inches, but if the output is reduced to 350 barrels daily the proper diameter 4
Williams, "Flow oi Gases in Furnaces," John Wiley & Sons, 1923.
April, 1929
INDUSTRIAL A N D ENGIh'EERIh'G CHEMISTRY
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The Efficient Rotary Kiln
The heat balance of the rotary kiln2 shows the possibilities for reducing fuel consumption. It is easy to save 100,000 B. t. u. per barrel or more by use of a regenerative cooler, which preheats the secondary air for combustion by abstracting the heat from the hot clinker. A small amount of heat can be saved by placing insulating brick back of the fire brick in the upper reaches of the kiln, but it is unsafe to insulate the hotter parts. The balance and major part of the saving must come from reclaiming a portion of the heat of the exit gases. smaller than demanded by theory, and the function of the stack is to provide enough draft to remove the gases from the stack base chamber, plus that extra amount of draft needed to speed the gases leaving the kiln beyond their natural velocity of free flow. Curve 1 could,
5 ,MD
0
3 f
setting of output.
TEMPERATURE WET PROCESS EXIT GAS.
OF.
Effect of Surdus Air
Figure 4 shows the effect upon fuel consumption of the observed kilns for various percentages of surplus air over that theoretically required for combustion. Surplus air is an important factor affecting fuel consumption and should be carried as low as possible. If combustion conditions are carefully controlled, it is possible to burn with practically no surplus air and with less than 1 per cent carbon monoxide. No cement plant should be operated without some means of controlling surplus air. Fuel Economy in Generation of Waste-Heat Steam
In computing the curve of Figure 5 we have chosen a kiln 8.5 feet inside diameter by 175 feet long because such a kiln is typical of waste-heat, boiler rotary-kiln installations. If all the steam required for power in a cement plant is to be derived from heat in the exit gases, these must contain from 600,000 to 700,000 B. t. u. of sensible heat and the output of the kiln will be between 820 and 940 barrels per day. The curve of Figure 6 is of particular significance in connection with waste-heat boiler practice. The sensible heat in the exit gases must be between 600,000 and 700,000 B. t. u. per barrel if all the steam for power generation is to be derived from this source in waste-heat boilers. The corresponding fuel consumption is from 1,350,000 to 1,450,000 13. t. u. per barrel. I n actual practice in dry process the latter figure is the more nearly correct. Obviously, the generation of sufficientwaste-heat steam to run the plant is not compatible with high fuel economy of the rotary kiln itself. Effect of Temperature of Exit Gases
The curves of Figure 7 apply to kilns using the wet process and show the relation between temperature of exit gas and fuel consumption for varying percentages of water in the slurry. For given temperatures of exit gas the percentage of water in the slurry has a very great effect on fuel consumption. These curves are independent of kiln size, diameter, or length. The curves of Figure 8 are for an 8.5-foot inside diameter, 225-foot long kiln using the wet process and show the relation between temperature of exit gas and output for different percentages of water in the slurry.
The most obvious step is to absorb this heat by the raw mix by increasing the length of the kiln, bearing in mind the important fact that a relation exists between diameter, length, fuel consumption, and output of clinker. Curve 2, Figure 9, shows the load characteristic curve of a 6 by 240 foot kiln with lower end expanded to 9.5 feet in diameter inside the brick. Such a kiln, when equipped with cooler and when using theoretical air, producing 1200 barrels daily, shows a fuel consumption of 1,050,000 B. t. u. per barrel. It seems safe to state that a consumption of 1,050,000 t o 1,100,000 B. t. u. is the ultimate economy for ordinary rotary kilns using the dry process.
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I N D USTRl-4L AND ENGINEERING CHEMISTRY
The theoretical fuel consumption of an ideal apparatus is 405,000 B. t. u. per barrel. The writer believes that an efficiency of 50 to 60 per cent should be attainable in practice by use of special rotary kilns, and this would correspond to a fuel consumption of 674,000 to 810,000 B. t. u. per barrel. I n Figure 9 are assembled three curves showing the output or fuel economy characteristics of three types of rotary kilns which are furnished with regenerative coolers that utilize the secondary air to abstract 100,000 B. t. u. per barrel from the hot clinker. A comparison of curves 2 and 1 shows the advantage of increasing the length of the kiln while keeping the diameter inside the brickwork constant. The longer kiln shows a better fuel economy a t all outputs and the fuel economy is less affected by output than is the case with the shorter kih. Curve 3 shows the curve for a special type of rotary kiln, proposed by the writer, in which devices are installed for improving heat transfer between the incoming raw mix and the outgoing gases in the upper 40 feet of the kiln. It is not the purpose at this time to describe the mechanical features of this improved type of kiln, but merely to call attention to the possibilities for greater fuel economy which result from effi-
Vol. 21, No. 4
cient countercurrent heat transfer between gas and raw mix in the upper end of the kiln. Figure 10 shows curves for a kiln of 8.5 feet inside diameter in its principal length by 240 feet long, and for the same kiln equipped with the special devices for improving countercurrent heat transfer in the upper end of the kiln proposed by the writer. These curves again bring out the fact that in the usual types of kilns of given length the 6-foot kiln has a better fuel economy than the 8.5-foot kiln; but the latter has a much greater output. The final fact which interests the engineer is actual commercial economy, and i t is quite possible that under usual conditions the 8.5 by 240 foot kiln will be more economical in a commercial sense than a 6 by 240 foot kiln. This is a problem involving all commercial factors, including cost of fuel, labor, power, money, and the respective investments per barrel per day of output. Such a study must pertain only to each special case, and is out of place here. The special device will give substantially the same fuel economy to the 8.5 by 240 foot kiln as to the 6 by 240 foot kiln, but the relative burden put upon these devices is greater in the former than the latter.
Methanol from Hydrogen and Carbon Monoxide 11-Dimethyl Ether1 Ralph L. Brown and A. E. Galloway PITTSBURGH
EXPERIMENT STATION, U.S. BUREAUOF MINES, PITTSBURGH,
PA.
