Rate of Drying Chrome Leather 0. A. HOUGEN, University of Wisconsin, Madison, Wis.
I
stage. In th? becond stage tLe K T H E process of chrome The general equation f o r the rate of drying rate of drying falls off directly l e a t h e r manufacture t h e chrome leather has been found experimentally to with decrease in the free moisture s k i n is d r i e d a f t e r t h e be : c o n t e n t of the stock. In the sequence of tanning, fat-liquorthird stage the rate of drying falls ing, and dyeing, and before the off more rapidly than the correfinishing operations of staking, sponding decrease in moisture tacking, graining, seasoning, and This equation holds for values of W below content. By u n i f o r m drying glazing. This intermediate dryconditions is meant that a t any ing operation is the one particu1.25 and for values of G above 10. For values location on the surface of the larly referred to in this discusof W above 1.25 the rate of drying is constant. leather the temperature of the sion. The purpose of this drying The average density of leather, excluding water, stock and the t e m p e r a t u r e , operation is to bring the tanned before drying is 0.46 and after drying is 0.50. humidity, velocity, and direction leather to a uniform state of dryThe methods of experimentally determining of the air stream remain constant. ness and to arrest all f u r t h e r During the first stage of drying chemical changes before completthe drying coeficients of leather and deriving the entire surface of the stock is ing the finishing operations of the above equation are described. The method of c o m p l e t e l y w e t t e d ; and, if improving the appearance and applying this equation io the progressive drying of evaporation proceeds adiabatihandle of the product. A f t e r leather is illustrated in problems for calculating cally, the surface attains the wet dyeing, the leather is hand-set. drying time, length of drier, optimum air velocity, bulb temperature of the air. In This operation consists of scrapthe second stage, evaporation is ing off all loose surface water and and the most economical fraction of recirculated a t the same time stretching the still restricted to the surface of air. leather in all directions t,o rethe stock, but the actual wetted move wrinkles and creases and to area decreases as drying proincrease area. Following drying, the leather is piled in stacks ceeds, thereby resulting in a decreased rate of drying. During and allowed t o remain “in crust” for a t least 12 hours. I n this this stage diffusion of water is so rapid as to maintain a nearly step the leather is cooled and regains moisture to reach equi- uniform moisture concentration, and the water a t the surlibrium with the atmosphere. The leather is then dipped in face assumes a submicroscopic network exposing an effective water for 10 minutes to produce a slight but not thorough wetted area proportional to the free moisture content of the wetting. The wetted leather is sammied overnight by piling stock. I n the third stage of drying the surface of evaporaand covering with wet burlap. This period effects a uniform tion extends into the interior in which case there is no free redistribution of water through the stock. After sammying, moisture a t the surface of the stock and vaporization prothe skins are staked by machine to stretch them in all direc- ceeds over an increasing zone of thickness. tions. The leather is then tacked on boards and dried before I n the particular problem of drying leather involved in seasoning. The stock is dried again after seasoning previous this investigation, only the second stage of drying need be to glazing and graining. This investigation deals solely with considered. The period of constant drying rate is avoided by the drying operation after dyeing and hand-setting and not the operation of hand-setting whereby all free surface water with the drying operation following tacking and after season- is scraped off. Wherever permissible this mechanical removal ing, although the same theory applies to all three drying of excess water is good practice. The mechanical removal of operations, and the same drying coefficients are perhaps water by hydroextracting, wringing, or scraping requires applicable. much less energy than by application of heat for evaporation. The usual method of drying chrome leather is by means of The thickness of chrome leather is relatively small compared a loft drier where the wet stock is suspended in a large room to the area exposed per pound of dry stock so that the resistand allowed to remain until the desired dryness is obtained, ance to diffusion of water is negligible compared to the drying being hastened by circulation of heated air. The resistance to diffusion of vapor into the surrounding air. modern trend in drying is toward the use of progressive The free moisture concentration of the stock remains nearly driers wherein the wet stock is passed continuously through uniform. Experimentally i t was found that under constant an enclosed tunnel countercurrent to a stream o f heated air. drying conditions the rate of drying decreased directly with The progressive drier has the advantages over a loft drier the decrease in the average free moisture content of the stock of being continuous in operation, requiring less handling, and inversely with the thickness of the stock, so that for all less labor, less space, less heat, and less time for drying, and of practical purposes the problem of drying chrome leather providing more nearly uniform drying conditions. The loft may be treated as one involving only the second stage of drier has the advantages of requiring less expensive equipment drying. All experimental data obtained can be satisfactorily and of being more flexible in output and in time allotted. interpreted on this basis so that even though an appreciable The original purpose of this investigation was to establish concentration gradient exists through the stock, and the data for the design of a progressive drier in order to replace surface of evaporation extends into the stock, no justificathe less economical and slower method of loft drying. tion is warranted in complicating the problem by consideration of these minor effects. Since the primary purpose of THEORY OF DRYING APPLIEDTO LEATHER this investigation was to obtain data for the design and operaThe drying of a fibrous material such as leather proceeds tion of progressive driers, i t is undesirable and entirely unin three stages. When dried under constant drying condi- necessary to increase unduly the complexity of matheinatica 1 tions, the rate of drying remains constant during the first treat men t . 333
INDUSTRIAL AND ENGINEERING CHEMISTRY
334
For this special case of drying, the fundamental drying equation reduces to the simple form derived by Walker, Lewis, and McAdams (6):
where T
=
Vol. 26, No. 3
W=T-E totaI moisture content
(3)
EXPERIMENTAL PROCEDURE dW - =
de
where B
=
-BWAH
mG*
= free moisture content of stock A H = unsaturation of air G = mass velocity of air flowing over sheets
W
thickness of stock time elapsed m,n = experimental constants L
e
It was the purpose of the experiments to establish values of the drying coefficient, B , to establish quantitatively the effect of air velocity, thickness of stock, and temperature of air upon this coefficient, and to determine the value of the constants, m and n, in Equation 2. An experimental drier was constructed large enough to dry thirty calfskins or kips.
= =
Sherwood (4) and later McCready and hfcCabe (2) have shown that, when water diffusion is the controlling factor, evaporation is not restricted to the surface of the sheet, but that the surface of evaporation extends inward as drying proceeds, and that the mechanism of drying then involves three resistances-the diffusion of water through the stock up to the evaporation zone, the diffusion of vapor through the evaporation zone, and the diffusion of vapor through the air film adjoining the surface of the solid. Furthermore, in the zone of vapor diffusion through the stock, evaporation proceeds through the entire thickness of this zone so that the Row of vapor increases as the outer surface is reached. The case of drying where liquid diffusion is the controlling factor and where vaporization occurs only a t the surface of the sheet has been fully treated by Sherwood (5) and Newman (3). EQUILIBRIUM MOISTURE COXTENT When a hygroscopic material such as leather is suspended in unsaturated air, the water content of the leather changes until it comes into equilibrium with the air. This equilibrium moisture content increases with the relative humidity of the air, decreases slightly with increase in temperature, and is also dependent upon whether the material is losing or gaining moisture. For drving problems it is important to know t h e equilibrium m o i s t u r e content when the stock is losing moisture, also called the “desorption moisture content.” I n plotting equilibrium moisture content, it is preferable to plot values against relative humidity rather than against perc e n t a g e humidity 3 since the former are a 3 independent of at$Q mospheric pressure. RELATIVE HUMIDITY V a l u e s of t h e FIGURE 1. EQUILIBRIUM MOISTURE d e s o r p t i o n moisOF CHHOME LEATHER(DE- t u r e c o n t e n t of CONTENT SORPT10.V VALUES) chrome leather are plotted in Figure 1 for both experimental and extrapolated values. In every case moisture content is expressed as a fraction of the weight of dry leather. The free moisture content can now be obtained from the relation :
c +
FIGURE 2. DIAGRAM OF EXPERIMENTAL DRIER d . Dry-bulb thermometer A . Heating coil B . Steam inlet automatic control valve C. Automatic temperature control thermometer D. Temperature recorder E. Humidity recorder F . Shutter (3. Blower w . Wet-bulb thermometer
H. J.
K.
L. M. N. 0.
P.
Motor Frame for supporting leather Platform scale Leather Inlet for fresh air Outlet for spent air Opening for recirculated air Live steam supply
This drier was of a compartment type, but, by recirculating a portion of the air, drying conditions could be made to simulate those in a progressive drier wherein the leather is progressively subjected to less humid air as drying proceeds. I n this way, practical drying conditions are simulated, in contrast to the frequently used method of employing constant drying conditions in laboratory experiments. Using progressively changing drying conditions during an experimental run complicates the calculations involved but yields more reliable information for practical operation and design. I n drying leather it is important to hang it in vertical sheets and to pass the air downward over the vertical surfaces rather than to blow the air horizontally. This is necessary because of the irregular shape and size of skins. If air were passed horizontally, the stream would channel through the open spaces of the bottom end without effective contact with the surface. I n a progressive drier for leather, air should be passed in a helical spiral through the drier, winding its way countercurrent to the travel of the stock, the air passing downward through the sheets of leather, leaving a t the bottom, and bypassing through side channels to the top of the next group of skins. Fans must be provided to effect each turn of the air stream, to maintain a helical flow, and to avoid a horizontal flow. A diagram of the experimental drier is shown in Figure 2. The temperature of the air entering the leather was controlled by automatic regulation of the steam supply to the heating coil by means of a Johnson thermoregulator. Each skin was hung from four supports with the butt ends up and adjustable to different spacings. A distance of 5 cm. apart was the minimum
March, 1934
INDUSTRIAL AND ENGINEERING
value found necessary to avoid touching and to allow a free passage of air. Fresh air was admitted through a port in the side. The proportion of fresh air admitted could be adjusted by varying the size of these port openings. A special intake at the front permitted admission of higher proportions of fresh air. Air was circulated by means of a blower and the rate of flow varied by changing the pulley ratios on the drive shaft. A continuous printed record of dry- and wet-bulb temperatures was registered by means of a Foxboro recording psychrometer. The leather was suspended on hooks fastened to a wooden frame, the whole of which was suspended from a platform scale mounted above the drier. By this scale a continuous record of the weight of leather and its moisture content could be obtained for the entire run. Wet- and dry-bulb thermometers, precise to 0.1' C., were placed for measuring the temperatures of fresh, spent, and recirculated air. The final moisture content of the leather after drying was obtained by cutting off samples for chemical analysis. Such samples were taken from various parts of the skin to determi,ne uniformity of drying and the average moisture content. The humidity of air was controlled by regulating 1 he supply of fresh air and also by admission of steam a t the beginning of the run. The fact that the drier was constructed of wood interfered with humidity control but did not affect the experimental values of drying coefficients. The velocity of air flowing downward between the skins was measured by an anemometer. Readings were taken a t twenty-one different points a t a horizontal elevation halfway down the skins. These velocities varied greatly from point to point owing to the irregularity of leather. From fifteen to thirty skins were taken for each test immediately after the operation of hand-setting. The following variations in drying conditions were tested: Average air velocities. 14 to 142 kg. per square meter per minute Temperature of saturated air, 26O to 60' C. Temperature of dry air, 26' to 93" C. Initial moisture content of leather, average 1.5 kg. per kg. of dry leather (60 per cent water) Final moisture content of leather, 0.025 to 0.155 kg. per kg. of dry leather (2.4 to 13.4 per cent water) Fraction of water removed, 9 0 to 98.5 per cent Time of drying, 2.5 to 18 hours Average thickness of stock, calfskins 1.03 mm., kipa 1.4 mm. Average dena3ty of leather excluding water, before drying 0.46, after drying 0.50
After drying, the thickness of each skin was measured a t five different places-namely, in the head, right butt, left butt, right brisket, and left brisket. The stock was placed "in crust" for a t least 12 hours. The dried leather was then dipped in water for 10 minutes. After dipping, the skins were passed through
I
/I
TIME Of
DRYlNB
IN
MINUTES
FIGURE3. EXPERIMENTAL DATAFOR RUN 16 Thickness of stock, 0.98 mm: temperature, 120" F. (48.9' 6.)
CHEMISTRY
335
the normal operations of sammying, staking, tacking, trimming, graining, and seasoning. Shrinkage of leather in drying was determined by measuring the area after hand-setting and after tacking. All area measurements were made by the same operator and on the same machine. The average shrinkage in area during drying was 5.7 per cent. Drying experiments were conducted under different conditions in order to ascertain the variation of the drying coefficient, B, with the variable factors such as temperature, air velocity, thickness of stock, and spacing of skins. Temperature readings of dry- and wet-bulb thermometers of the fresh, recirculated, and spent air were taken in duplicate every 10 minutes. To compensate for fluctuations in temperature due to operation of the thermoregulator, the temperature readings were taken in duplicate a t half-cycle intervals of the periodic operation of heater. For instance, if the thermoregulator operated every 2 minutes, readings were taken 1 minute apart for each 10-minute interval. Average of duplicate readings were used in calculations. The period of fluctuation increased as the leather approached dryness. The weight of leather was also recorded a t io-minute intervals. In startine a test the drier was first heated to the desired temperature, a
