Drying and Curing Bright Leaf Tobacco with
Conditioned Air ALBERT H. COOPER, CARL D. DELAMAR, AND HENRY B. SMITH Virginia Polytechnic Institute, Blacksburg, Va.
The flue-curing process for tobacco is practiced today in nearly t h e same manner as when fluecured tobacco first became of commercial importance; no definite procedure is followed. A scientific study has proved t h a t t h e curing process is defined within narrow limits, and t h a t control, rather than art, may be relied upon more definitely. Tobacco curing differs from a true drying problem in that both physical and chemical changes are involved. Since it is impractical to evaluate the chemical changes because of their complexity, in this work the physical conditions were investigated, for they control both the physical and the chemical changes which take place. Drying rate curves for each period of the curing process were obtained over a wide range of constant conditions of temperature, humidity, and air velocity. Correlation of these curing curves indicate narrow limits of conditions for satisfactory curing and critical points beyond which poor quality of tobacco may result. Air conditioning improves the process by reducing the total time approximately one half and thus doubling the capacity of the barn, by producing tobacco of uniform quality and thus largely reducing the loss from improper curing, and by lowering labor and fuel requirements considerably.
A TYPICAL FLUE-CURING BARNFOR BRIGHTLEAF To(above), AND A MODERNAIR-CONDITIONED INSULATED BARN(below)
BACCO
L
ITTLE improvement has been made in the methods of curing bright leaf tobacco since tobacco first became of commercial importance. Flue curing is carried out in practically the same manner as when it was originally developed. There are no set rules, but each grower usually employs certain procedures which have been handed down by his predecessors clr is guided by his own past experiences. The tobacco plant has been improved by the agronomist to such an extent that proper cultivation yields excellent crops, but with present)-day methods of curing and drying, the quality of the cured tobacco may be greatly lowered by the conditions of curing. The curing barns are, in general, crude and have extremely poor methods of heat transfer and distribution, as well as poor control of the moisture content of the inside air. A large amount of tobacco is lost each year because of the inefficiency of the operations and uncontrolled conditions of temperature and humidity. I n addition, there is an appreciable fire hazard as a result of hot furnaces, of overheated flues (3, 6) or of contact of flues with the wooden structures or the dried tobacco within the barn.
from green to lemon yellow (6). During this process, elimination of water of the tobacco leaf must be controlled since water is essential for the life of the plant and these processes of conversion (8). Too rapid elimination of water causes the plant cells t o die prematurely, and the conversion of the chlorophyll is prevented (8). Finally the process results in B purely drying stage in which the bound water is removed
Purpose and Scope of Investigation Tobacco curing differs from a true drying problem in that both chemical and physical changes are involved. Curing is a physiological process (4) in which the stored-up food of the plant cells is gradually consumed during the curing process, while the chlorophyll changes and results in color changes
V,11).
The chemical reactions involved in such a physiological process are highly complex (5, 6) and it is impractical to evaluate them for control purposes. The two factors that 194
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FEBRUARY, 1940
INDUSTRIAL AND ENGINEERING CHEMISTRY
control the curing process are temperature and drying rate ( I S ) . Drying rates depend upon temperature, humidity, and, to some extent, upon air velocity. I n this work the physical factors were investigated, for they control both the physical and chemical changes which take place. The objectives of this investigation have been t o cure bright leaf tobacco from the green (usually termed “ripe”) to the optimum cured state with controlled conditions and to determine the effect of these variables. By means of the variable factors-temperature, humidity, and air velocitythe limits of satisfactory conditions and the conditions which are most favorable to the curing of such tobacco are studied.
195
The temperature required for stem drying is relatively high, and since the curing proper has been completed, no harmis done in using more extreme conditions.
The Flue-Curing Process Actual curing of bright leaf tobacco is carried out in three distinct stages (6)-the “yellowing” period, the “fixing” period, and the “killing” period. The complex physiological (15) processes taking place in the first, or yellowing, period of curing result in color changes from green to lemon yellow. This is the most important period of the entire curing process for the quality of the tobacco is chiefly determined at this time (IS). The yellowing period takes place while the leaf is still alive ( d ) , and thus the processes are living or vital; any factor that destroys the plant life prematurely leads to tobacco of poor quality. The two factors that control this process are temperature and drying rate. After the yellow color of the leaf has developed, temperature conditions are changed to bring to completion the physiological processes involved in the curing. I n this second, or fixing, period the life of the leaf is completely destroyed. The completion of these two periods is determined by the shade of color. The temperatures are maintained low in the yellowing period and are increased in the subsequent periods. Little is done to control humidity in any of the periods, but it is generally high in the yellowing period and lower in the second and third periods. The third, or killing, period is characterized by high temperatures and relatively low humidities. The physiological processes of curing are complete, and this period is a true drying stage. At this point the leaf may be considered killed, the major p o r t i o n of t h e original water c o n t e n t of t h e leaves is removed, and the web part of the leaves is left essentially dry. However, the stems and the larger vesicles of the leaves still retain a large p r o p o r t i o n of their water content. Conseq u e n t l y , if they are not thoroughly CENTER LEAF dried, they will T O P LEAF CROP MEAN continue to feed water to the leaf structure and thus e n c o u r a g e BONE-DRY WEIGHT- G H S rotting and fungus growth. FIGURE1
4
0
3
6
8
I2
15
18
21 24 2 7 30 TIME HOURS
-
33
36
39
42
45
A I
FIGURE 2
At the end of the curing and drying process, the tobacco leaves are extremely brittle, and seriom damage would result upon handling and packing. I n the fourth, or “ordering”, period, the leaves are allowed to absorb moisture from the air; they thus become pliable and can be handled easily. This is usually accomplished by opening the barn t o cool and allowing fresh humid air t o come into contact with the leaves. This period is long unless the weather is damp.
Experimental Procedure I n this study the physical conditions of curing were investigated-namely, the relation between water content and the drying rate-for they control both physical and chemical changes. The effect of three variables (temperature, humid. ity, and air velocity) on the drying rates and curing procew were studied : Tobacco was cured under a wide range of constant and controlled conditions in an experimental air-conditioned drying unit. The dr er had a capacity of two hundred leaves and was equipped with t8ermostatic regulators. These regulators controlled the temperature in the dryer within 1’ F. and humidity to a maximum variation of * I per rent. The dryer mas constructed of steel angle-iron frame, covered with galvanized sheet iron and Celotex. Air taken in by a s uirrel cage type blower was passed through a tempering unit, wlere the temperature was raised to a ydetermined point in order to humidify properly, and was t en passed through a humidification compartment. This compartment was equipped with six spra nozzles and a centrifugal pump for recirculation of the water. Jrovision was also made for heating the spray water to any desired temperature by a tempering coil or to introduce live steam direct into the sprays. After being humidified, the air was forced through a heating unit where the final temperature was adjusted before it was introduced into the curing or drying compartment. By means of a damper arrangement air was completely recirculated when high humidity was desired; for partial recirculation air was used in one pass only, and when low humidity was desired, fresh air was used entirely. Tobacco, supported in the drying unit on racks, was sus ended by a steel frame to a balance located on top of the unit. &eight readings were recorded a t hour intervals throughout the curing and dryin! y c e s s . Quality of the tobacco cured was also judged, an t e limits of these variables were determined for good curing. Drying rates were studied under a wide range of constant conditions of temperature, humidity, and air velocity for each period of the entire curing and drying process. Correlation of these data for a series of curing and drying curves was attempted by a study of average rates over comparative periods in the process. Data were correlated for tobacco from different crops and s e e sons, and were found to be in excellent agreement.
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Basis for Drying Rates Drying rates based on the usual surface area were deemed impractical because of the irregular shape of the tobacco leaves; therefore a study of the relation between the weights of the green and of the dried leaves was made and curves were developed. Figure 1 illustrates the straighbline functions obtained. I n view of the constant relations found, these dry weights were used as a basis for establishing the drying rate curves. The means are represented by the following equations: Center leaf mean: T
=
6.64 E
Top leaf mean: 5" = 5.675 E
-
Over-all mean: T = 6.075 E where T = total green weight E = dry equilibrium weight, after drying at 180' F. and 3% relative humidity (1) The latter of the equations was used in determining the dry weight for the drying rate curves.
Curing and Drying Curves For a given set of air conditions there are three variables which are mutual functions-namely, time, water content, and drying rate. The method used in presenting these variables consists of two curves-water content versus time, and water content versus drying rate. Figure 2 shows the relation between water content and time. The effect of the transition from one period to the other is clearly shown by the sudden breaks in the curve. Best results have been obtained in starting the second period after the water content reaches 2 or 3-.grams water per gram dry weight. However, this point is also a function of time and temperature, since proper curing must take place. The third period may be considered a true drying problem; and the remaining water, a large part of which is in the stem, can be removed as rapidly as possible. However, it is not well to subject the tobacco to temperatures above 180" F. because some kind of physiological action in the stem p. 0.6 takes place (drivP > ingoff certainprodU ucts) which is not conducive to best @ TEMPERATURE taste and odor in @ TEMPERATURE-IOS*L the leaf. The last I
stage Of the pro+ ess,ordering,isrepresented bya nega0 h G rate* 0.2 The curve shows that a regain (I6) Of water to a 0.1 Of 28 per cent was more than suffi0 cient to render the 0 a5 1.0 ~5 20 2.5 3.0 35 40 45 5.0 leaf pliable enough WATER CONTENT--CMS,/GM. DRY WT. to be FIGURE 3 dled. The drying rate is expressed by the usual ordinates of water content us. loss of water, expressed on some representative basis. Figure 3 shows a typical curve in terms of water content and drying rate. On this basis the rate of drying in the first period is relatively low. This curve also shows clearly the increase in the rate of drying for changes in humidity and temperature. Y
$
0.3
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Temperature Table I1 and Figure 4 show the effect of temperature on the drying rate during the first period of curing. I n the proximity of 1 1 2 O F. there is a change of slope, above which drying
TABLE I. DRYING RATEOF TOBACCO rveight
~i~~
Weight Loss
Total
Water
Content G./o.
Drying Rate
Grams Grams Grams (3./dhr. First Period: 95' F. Dry-Bulb, 91.5' F. Wet-Bulb, 85% Relative Humidity 0.0 4322 0.0 3610 5.075
Hours
0.58
0.92 1.25 1.75 2.25 2.75 3.25 3 75 4.75 5.75 6.75 7.75 9.75 10.75 12.00
i::
14.75 15.75 17.00 17.75 18.75 21.75 22, 75 23.75 24.75 25,75 26.75 28,00
Second
3,569 5.020 0.0987 3.548 4.980 0 0888 3.523 4,960 1.0550 3488 4 905 0,0984 3468 4,870 0.0563 3423 4.820 0.0984 3393 4.770 0.0844 3368 4 730 0.0703 3289 4.620 0.1110 3223 4.530 0.0927 3145 4 425 0.1095 3078 4.330 0.0942 4.160 0,0953 2957 2893 4 070 o om0 2826 3,975 0,0753 2781 3.910 0 0843 2730 3 840 0.0717 3378 2666 3.750 0 OR93 3326 2614 3 675 0.0732 2555 3,590 0.0664 3267 0 0694 3230 2.518 3 540 2468 3,475 0.0703 3180 2429 3 415 0.0604 0 0604 2392 3,353 3053 2341 3.290 0.0.577 3010 2298 3.230 0.0604 2968 2256 3.170 0 0590 2928 2216 3 110 O.OM3 2883 2171 3 050 0.0632 2849 2137 3.005 0.0478 2soo 2088 2.940 0.0551 Period: 112' F. Dry-Bulb, 92' F. Wet-Bulb, 45% Relative
zt:;: $:!:,
4281 4260 4235 4200 4180 4135 4105 4080 4001 3935 3857 3790 3669 3605 3538
ih:l
41 21 25 35 20 35 30 25 79 66 78 67 121 64 67 45 51 64 52 59 37 50 43 43 41 43 42 40 45 34 49
I
Humidity
i:7i
117 1971 2.773 0.2190 173 1798 2.530 0.2430 457 1341 1.888 0,1593 57 1294 1.806 0.0802 35.83 1935 61 1223 1.720 0.0791 49 1174 1.652 0.0752 42 1132 1.593 0.0590 40 1092 1.537 0.0563 13 1079 1.520 0.0366 Third Period: 180' F. Dry-Bulb, 102' F. Wet-Bulb 39.75 1608 183 896 1.257 0.5140 88 808 1.135 0.4940 176 632 0.888 0.3095 41.25 1210 134 499 0.701 0 4180 41,78 1105 105 393 0.553 0 2770 42.35 980 125 268 0.376 0.3780 42.95 952 28 240 0.338 0.0787 43.35 920 32 208 0.292 0.0900 43.85 913 7 201 0.282 0.0197 Ordering Period: 115" F. Dry-Bulb, 105' F. Wet-Bulb 44,18 958 -45 246 0.346 -0.1900 44.52 990 42 27s 0.391 -0.1772 44.85 'Oo0 - 10 288 0.405 -0.0468 ~~~~
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;2:
~~~~
-
rates increase rapidly. Observation of the physical appearance of the leaves indicated that the drying rate was too rapid and that possibly thermal decomposition was occurring simultaneously. Above this critical temperature the quality of the cured tobacco leaf is lowered and no yellowing occurs, the green color of the leaf being fixed. Below this temperature range excellent curing results, and the most effective safe range is found to be between 00' and 105' F. The critical temperature range may be explained by the fact that the hydrolysis of starch is accelerated (a), but that a t the same time drying is taking place a t such a rate that the vital cell process is destroyed as a result of the rapid loss of water from the plant cells and protoplasm. Figure 5 shows excessive drying rates during the early stages, with a decrease in rate in subsequent time intervals. O F TEMPERATURE ON DRYING RATE' TABLE 11. EFFECT
Dry-Bulb Temp,
Av. Wt. Loss Bone-Dry Wt. Av. Drying Rate Q./hr. Grams a./o./hr. 0.510 8.00 0.0636 62.70 712.00 0,0881 0,630 5.10 0.1265 1.720 7.90 0.2165 1.985 6.10 0.3254 Relative humidity, 85 per cent; air velocity, 50 feet per minute. F. 90 95 100 110 115
O
0
FEBRUARY, 1940
INDUSTRIPlL AND ENGINEERING CHEMISTRY
197 21
.I9
c
u
$ .I7 OL >
9
15
a
$ L
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I Y c
a
90
95
100 IO5 TEMPERATURE,
110
.I(
a
115
.09
Or.
FIGURE5
60
90
100
,051 70
I20
110
I
1
I
Humidity I n contrast to normal drying processes, high humidities are required in two phases of the curing of tobacco. I n the yellowing period it is necessary to have a relative humidity high enough to prevent too rapid desiccation of the leaf, for this alone will ensure the curing process. Even a t normal atmospheric temperatures and humidities, drying proceeds too rapidly. Table I11 and Figure 6 show the effect of humidity during the first period, I n the range of 72 to 80 per cent relative humidity there is a general change in the slope of the curve. Similar to the critical temperature range, above this humidity there was excellent curing; below this range poor curing was obtained and inferior tobacco resulted. The lower humidities apparently caused a too rapid reduction of the water content of the plant cells below the minimum value necessary for the physiological processes involved in curing. The curves for different intervals of time (Figure 7 ) indicate that relative humidity plays an important part in initial stages of the process but has a decreasing effect as curing proceeds. I n order to obtain good curing, the relative humidity of the air must be in the range of 80 to 85 per cent during the initial stages of curing. The results emphasize the need for careful control of hu14 0 3 - 0 HR PERIOD + 8 9-12 HR PERIOD midity in the earlier 0 0 15-18 HR PERIOD stages of curing. a21 -24 HR PERIOD
:
12
Since the entire process occurs in the falling rate period of drying (10),it maybe expected that air velocity does not play an important part in the drying rate. Diffusion of moisture to the surface is the controlling factor (1.2). The chief function of the air velocity is to establish constant and
I
I
1
I
FIGURE6
FIGURE4
Air Velocity
I
I
80 85 90 95 RELATIVE HUMIDITY -PER C E N T
15
TEUPERATURE,'F
&
uniform conditions of temperature and humidity. Table I V and Figure 8 show the effect of air velocity on drying rate. I n approaching the low velocity, in the range occurring in the tobacco barns due to natural convection, close control of temperature and humidity is extremely difficult; and it is practically impossible to obtain uniform conditions throughout the barn. The limits of air velocity are established by the fact that excessive leaf vibration is encountered a t a velocity above approximately 1 foot per second. The lower limit is a function of the depth of the packing and the drying rate, since the velocity must be sufficient to prevent excessive changes of humidity and temperature as the air moves
TABLE111. EFFECTOF HUMIDITY ON DRYING RATE" Relative Humidity
Av. Wt. Loss
P e r cent 75 80 85 90
G./hr. 131.2 46.7 50.1 45.2
a
G./g./hr. 0.1847 0. 1143 0,0907 0.0760
TABLE IV. EFFECTOF AIR VELOCITYON DRYING RATE' AV.
Velocity Ft./min. 4.75 12.39 18.51 23.53 30.10 30.10 40.55
dV.
Wt. Loss
Dry Weight
G./hr. 0.54 1.02 1.15 0.95 0.94 1.18 1.10
Grams 5.69 9.70 11.65 7.70 6.72 8.05 7.70
Drying Rate
Wt. Dry Velocity Loss Weight G./Q./hr. Ft./min. G . / h r . Grams
Drying Rate
0.0940 0.1050 0,099 0.123 0,139 0.147 0.143
G./g./hr. 0.137 0.159 0.1450 0,1360 0.1521 0.1643 0.15G4
50.50 50.50 50.00 78.00 96.00 100.00 115.00
0.89 1.36 33.40 75.10 31.10 31.20 30.10
6.50 8..55 230.00 532.30 204.00 190 00 192.50
Dry-bulb temperature, 100' F.; net-bulb, 9 6 O F.; relative humidity,
85 per cent.
.IO
d
5j
Grama 712.0 407.0 553.0 595.0
Temperature, 95' F.; air velocity, 50 feet per minute.
d
q
Bone-Dry W t . , Av. Drying Rate
-
a8
I
u u
I-
a ce 0
z a
54
.IS
nF K6
J2
.04
70
75
80
85
RELATIVE HUMIDITY -PPLR
FIGURE 7
so CENT
0
14
PI
42 58 70 84 A I R VLLOCITY- F T / M I N
FIGURE8
OS
112
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INDUSTRIAL AND ENGIKEERIKG CHEMISTRY
through the barn. I n the drying unit used in this investigation 20 feet per minute was found to be the minimum satisfactory air velocity.
Conclusions The application of air conditioning to the curing and drying of bright leaf tobacco improves the process in the follo\ving ways : 1. The time of curing and drying is reduced ap roximately 50 per cent. The yellowing period is not materially s f h e n e d , since this rate depends upon a physiolo ical proress. The fixing period is reduced considerably, and thetilling (9) and ordering periods are shortened to a large extent. 2. Optimum atmospheric conditions favorable to excellent curing are closely controlled, and air may be distributed uniformly throughout the barn. There is no loss of tobacco due to excessive temperature or uncontrolled humidity, and a more uniformly high quality may be obtained. 3. Labor and fuel requirements are considerably reduced. Automatic control leave the farmer free for other duties. Present methods require considerable day and night observation. A large portion of the fuel is consumed in the barn during the killing period, where high temperatures are maintained for considerable time. This consumption of fuel should be greatly lowered as a result of the shortening of this period. 4. Increase of the capacity of the barn, as a result of the shortened process time, will reduce the investment and maintenance cost. This method indicates possibilities of community operation in contrast to the large number of individually owned barns. 5 . The fire hazard of hot flues within the barn is reduced.
VOL. 32, NO, 2
Bibliography (1) Badger, W. L., and McCabe, W. L., “Elements of Chemical Engineering”, 2nd ed., p . 298, New York, McGraw-Hill Book Co., 1936. ( 2 ) Darkis, F. R., Dixon, L. F., Wolf, F. A., and Gross, P. M., IND. ENG.CHEM.,28, 1214-23 (1936). (3) Delamar, C. D., thesis, Va. Polytechnic I n s t . , 1938. (4) Garner, W. W., U. S. Dept. Agr., Farmer’s Bull. 523 (July, 192.1). ~ . - _ Garner, W. W., Bacon, C. W., and Bowling, J. D., Jr., ISD.ENQ. CHEM.,26, 970-4 (1934). Killebrew. J. B., and Myrick, H., “Tobacco Leaf”, pp. 209-32, New York, Orange J u d d Co., 1920. McCready. D. W., and McCabe, W.L., Trans. Am. Inst. Chem. Engrs.. 29, 131-59 (1933). Maxirnov, N . A., “Textbook of P l a n t Physiology”, New York, McGraw-Hill Book Co , 1930. Sherwood, T. K., Chemical Engineers’ Handbook, p. 1260, New York, McGraw-Hill Book Co., 1934. Sherwood, T. K., Trans. Am. Inst. Chem. Engrs., 23, 28-44 (1929). Sherwood, T. K., Ibid., 32, 150-68 (1936). Sherwood, T. K., and Comings, E. W., IXD.E m . C H m f . , 26, 1096-8 (1934). Smith, H. B., thesis, Va. Polytechnic Inst., 1939. Stillwell, S. T. C , Trans. Inst. Chem. Engrs (London), 6 , 91-101 (1928). \----. Thatcher, R. W . ,, “Chemistry of P l a n t Life”, New York, McGraw-Hill Rook Co., 1921. Walker, W.H., Lewis, W.K., MoA4dams,W. H., and Gilliland, E. R., “Principles of Chemical Engineering”, 3rd ed., p. 642, Kew York, McGraw-Hill Book Co., 1937. PRESENTED before the Division of Industrial and Engineering Chemistry a4 the 97th Meeting of the American Chemical Society, Baltimore, Md.
The System Trisodium PhosphateSodium Carbonate- Water KENNETH A. KOBE AND ALEXANDER LEIPPER University of Washington, Seattle, Wash.
HE solubility data for trisodium phosphate in water disagree badly, and no data exist for the system trisodium phosphate-sodium carbonate-water. The purpose of this paper is to present the data for this ternary system, which will include the binary system trisodium phosphate-water, from the cryohydric point to 100’ C. The reasons for disagreement among the various solubility determinations will be given. It is well known that in commercial trisodium phosphate a certain amount of excess sodium hydroxide is present which will give a formula approximately represented by Na8PO4.l/?NaOH with 10 or 12 molecules of water of crystallization. I n their study of the system Na20-P206-H20, D’Ans and Schreiner (3) state that one of the solid phases is Na3P04.12HzO, but their data are not clear. They report that the crystals were analyzed after being dried between porous plates. The analytical method is of importance so that the excess alkali may be determined. If the crystals are Na3P04.12H20, the P206content is 18.7 per cent, whereas Na,P04.1/7 NaOH.12 HzO has 18.4 per cent PzOs. This difference is so small that it could readily be attributed to imperfect
T
drying of the crystals. Since it is known that commercial trisodium phosphate (t. s. p.) does crystallize with an excess of alkali, any analysis of the solid phase must consist of a determination of both the PzOsand KazO content to determine their ratio in the crystal. Extensive experiments were performed by Smith (16) to prepare pure trisodium phosphate free from excess alkali. Using theoretical proportions of phosphoric acid and sodium hydroxide, he found that the first crop of crystals representing about 25 per cent of the total P20bhad the composition 2Na3P04.Sa2HP04. K i t h 4 per cent excess sodium hydroxide crystals were obtained which had the composition 171/2Na3P04.Ka20. As a result of his work, Smith concluded that either trisodium phosphate cannot be crystallized from its components or else it does not exist under norma1 conditions. hlenzel and von Sahr (9) studied in detail the composition of the solid phase in equilibrium with the saturated solutions in the system Na20-P2OrH20. Beginning a t the transition point of the dibasic to tribasic phosphate where the ratio of Na20 to P20sin the solution is 2.67, the solid phase shows
