April, 1929
INDUSTRIAL A N D ENGINEERING CHEMISTRY
305
Hygroscopicity of Fertilizer Materials and Mixtures' . I . Richard Adams and Albert R. Merz BURSAD 0s CHESISTBY 1ND SOZLS,
WAsnrlicioh., D. C
O M P A R I S O N of the Different materials can be The vapor pressures of saturated solutions of fertilizer present grades of fersalts have been determined at intervals from 10" t o compared most conveniently t i l i z e r s w i t h those 50" C., and calculations made of the relative humidities by this method when a11 are prevailing over the solutions. It has been found that manufactured a few years ago exposed simultaneously in the the concentrated fertilizer Salts, potassium nitrate, bunridity chambers. Itisimdiscloses a well-marked tendency toward an increase in ammonium phosphate, and Potassium phosphate, are portant that all samples a p concentration. T h i s tendamong t h e least hygroscopic fertilizer salts. The proximate a uniform weight h?groscopicity of a mixture of fertilizer salts is, in encyisaresultof thedevelopand that all be ground to a m e n t of processes for the similar degree of fineness. general, greater than t h a t of the more hygroscopic production of fertilizer mateThe relative hygroscopieities salt, though in some cases the h?groscopicity of the rials from the nitrogen of the of the materials will then be mixture is less than t h a t of the more hygroscopic salt. air and the combination or These determinations are of value in predicting the in the order in which tire storage and handling qualities of fertilizer salt samples become moist, mixture of these m a t e r i a l s with concentrated pr0duct.s mixtures. The results given in this of phosphoric acid and potash paper are based on the more salts. accurate procedure of measuring the vapor pressure of the With increase in concentration a number of properties saturated solution of the material under emnunation. The which required but little consideration in the case of ordinary determinations were made by a modification of the static fertilizers now demand attention. Among these properties is method of Smith and Menzies.* With the exception of the that of hygroscopicity. The hygroscopicity of a fertilizer monocalcium phosphate, all the materials were purified by affects such important qunlities as it,sdrillability, capacity for recrystallization from a saturated solution of the chemically storage, action on bags, and suitability for shipment. pure salt. The monocalcium phosphate was purified by exIt is well known that at a definite temperature the vapor traction of the commercially pure salt with ether. The pressure of the aqueous solution of a non-volatile soluble equipnient used is represented in Figures 1 and 2. substance is less than that of pure water. This decrease of Apparatus and Procedure vapor pressure becomes greater with increase in concentration of the solution. When such solid is exposed to an atmosThe substance under investigation is placed with its satuphere having a partial pressure of water greater than that of rated solution in a small glass bulb, A (Figure 2 ) . Tbis is its saturated solution, moisture will condense on its surface sealed t o the U-tube, €2, in which a confining liquid with an to form a saturated solution. This condensation of moisture inappreciable vapor pressure is contained. The bulb and from the air will continue as long as the partial aqueousva- U-tube form a unit known as an isoteniscooe frnrn the faat, por pressure of the latter exceeds that of the saturated solution or until the substance is entirely dissolved, and the resultant solution reaches a dilution whose vapor pressure equals that prevailing in the atmosphere. This tendency of a substance to absorb water from the atmosphere is directly proportional to the difference between the vapor pressure of its saturated solution and that of water. The re1at.ive hygroscopicities of various substances mny be determined, therefore, by measuring the vapor premure of their saturated solutions. A simple method for roughly deterniining the relative hygroscopicity of a material consists in exposing it to atmospheres of known relativo humidities in R series of constanthumidity chambers. These are most conveniently prepared by placing saturated solutions of salts which give the desired humidities in a series of desiccators with an excess of the salt t o insure saturation. Small portions of the material whose hygroscopicity is to be determined are then spread in thin layers on watch glasses or other shallow vessels and placed FiCure I-Apparatus for Determination of Hyprascopkify in the humidity cha.mbers for a period of time. The material of Fertilizers takes uu moisture in those chambers in which the humidity
I N D U S T R I A L A N D ENGINEERIXG CHEMISTRY
306
Table I-Relative
1
Hygroscopicities of Fertilizer Materials
VAPORPRESSURE OF SATURATED SOLUTION AT:
MATERIALS
Ca(hr03)z.4Hn0 "4hr03
NaC! Nah'Oa NHiC! (NHa)ZSO4 CO(NHz)n , KC! KNOi NHdHzPO4 C~HI(PO~)Z.HZO KHnPOi &SO4
IOo C.
15O C.
20°C.
Mm.
Mm. 7.16 8.95 9.87 9.85 10.15 10.16 10.24 11.05 12.26 12.44 12.67 12.62 12.78
9.73 11,74 13.63 13.53 13,92 14,22 14.05 15.04 16.21 16.10 16.52 16.89 17.30
6:88 7.00 7.13 7.27 7.29 7.48 8.07 8.87 8.94 8.95 8.96 9.06
Mm.
25'C.
30° C.
40'C.
50" C.
Mnr. 12.04 14.94 18.01 17.73 18.12 19.50 18.06 19.89 21.94 21.91 22.89 22.76 23.56
Mm.
14.88 18.93 23.96 23.07 24.61 25.22 23.09 26.75 28.84 29.18 29. 85 29.60 30.68
Mm. 19.68 29.11 41.37 38.81 40.81 43.32 37.66 44.99 48.67 50.05 52.37 51.46 53.04
44171 68.50 62.21 65,92 71.93 57.77 73.97 78.56 81.56 87.50 85.63 88.57
bottles, D and F , are placed in the system in order that the admission or withdrawal of small amounts of air will cause but slight changes in the pressure within the system. This facilitates accurate adjustment of the pressure. The threeway stopcock, B, permits evacuation by means of a vacuum pump or the admission of air through the glass vessel, G. The system is first evacuated with the cock I closed and cock K open. K is then closed and cock I opened. The reduction of pressure in the isoteniscope causes the solution to boil,
Figure 2-Diagram
VOl. 21, N o . 4
U
of Apparatus for Determination of frygroscopicity of Fertilizers
which drives out all air contained within the bulb. The pressure in the bulb is then the vapor pressure of the saturated solution alone. Air is then admitted by means of cocks J and E until the liquid levels in the U-tube of the isoteniscope are the same. If slightly too much air has been admitted, the cock E may be turned so as to connect again with the vacuum reservoir, F. When the levels are adjusted the pressure is then read on the manometer. To insure uniformity of temperature the isoteniscope is immersed in the thermostat, C. Vapor Pressures of Fertilizer Materials
Table I gives the vapor pressures of the saturated solutions of various fertilizer salts over a range of temperatures from 10" to 50" C. The vapor pressure of a saturated solution, when divided by the partial pressure of the aqueous vapor in an atmosphere saturated with water a t the same temperature and multiplied by 100, gives the relative humidity (expressed in percentage) of the air in equilibrium with the saturated solution. When the relative humidity of the air exceeds this, the material will take up water, and when the relative humidity of the air is lese, water is given up to it by the saturated solution. The relative humidities corresponding to the vapor pressures listed are given in the last seven columns.
Mm.
HUMIDITY OF AIR
1 %
10" C. ,I
75:3 76.6 78.0 79.5 79.8 81.8 88.3 97.0 97.8 97.9 98.0 99.1
15OC.
IN
20°C.
EOUILIBRIUM WITH
SATURATED
SOCUTION 25OC.
3OoC.
40" C. 50' C.
%
a
T " ,"
a
%
55.9 69.8 77.0 76.8 79.2 79.3 79.9 86.2 95.6 97.0 98.8 98.4 99.7
55.4 66.9 77.6 77.1 79.3 81.0 80.0 85.7 92.3 91.7 94.1 96.2 98.5
50.5 62.7 75.5 74.4 76.0 81.8 75.8 83.4 92.0 91.9 96.0 95.4 98.8
46.7 59.4 75.2 72.4 77.2 79.2 72.5 84.0 90.5 91.6 93.7 92.9 96.3
35.5 52.5 74.7 70.1 73.7 78.2 68.0 81.2 87.9 90.3 94.5 92.9 95.7
,I
I _
I"
%
I"
41:4
74.1 67.3 71.3 77.8 62.5 80.0 85.0 88.2 94.6 92.6 95.8
From the table it may be seen that, in general, salts increase in hygroscopicity with rise of temperature. It is also evident that the nitrogen fixation products, calcium nitrate and ammonium nitrate, as well as urea, are among the most hygroscopic of the materials examined. On the contrary, the two-fertilizer-component salts, monoammonium phosphate, monopotassium phosphate, and potassium nitrate, are among the least hygroscopic materials. Vapor Pressure of Fertilizer Mixtures
Table I1 gives the results of vapor pressure determinations on mixtures. As a rule the hygroscopicity of a mixture of two materials is greater than that of the more hygroscopic constituent. I n certain cases, however, the two materials may react t o form a compound or double salt which gives a greater vapor pressure in solution than that of the more hygroscopic constituent. The hygroscopicity of Such a mixture will then depend upon whether the proportion of the constituents is such as to give the compound alone or in mixture with an excess of either constituent. Thus, calcium nitrate and urea combine to form the compound Ca(h'O&.4CO(NH2)2,which is sold commercially under the name Calurea, This is decidedly less hygroscopic than calcium nitrate. Table 11-Vapor Pressures of Saturated Solutions of Mixtures a t 30' C.
pz&
' RELATIVE
HUMIDITY OF
MATERIALS
AIR IN
EQUILIBRIUM
W Y I T HSATD.
5.75 7.48 10.00 12.00 14.49 14.56 17.99 21.25 14.71 14.75 16.38 16.82 17.95 18.42 18.46 19.07 19.20 19.82 20.55 20.73 20.77 20.77 21.53 21.91 22.32 22.70 22.77 22.98 23.39 23.68 24.13 25.01 25.92 26.48 26.55 27.97 28,26 28.71 28.82 28.99 29.86
18.1 23.5 31.4 37.7 45.6 45.7 56.5 66.7 46.2 46.3 51.4 52.8 56.4 57.9 58.0 59.9 60.3 62.3 64.5 65.1 65.2 65.2 67.6 68.8 70.1 71.3 71.5 72.2 73.5 74.4 75.8 78.6 81.4 83.2 83.4 87.8 88.8 90.1 90.5
91.0 93.8
SOLN.
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
307
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 it 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; at 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 at 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-
