978
A C
= interfacial arra = concentration of solute
D
= distribution coefficient
k
= individual film coefficicnt
INDUSTRIAL AND ENGINEERING CHEMISTRY
K = over-all film coefficient V = volume of solution V7 = weight of solute transfprred
e
= time
Subscripts
R, E = respective liquid phases i = interface between phases
Vol. 41, No. 5
Supelscripts 0 = initial condition * = concentration needed in one solution t o bc at equilibrium IT ith serond solution LITERATURE CLTEU
(1) Hixson and Bsurn, IXD.EXG.CHEIM.,33, 478 (1941); 34, 120 (1942); 36, 528 (1944). (2) Hixson and Croneil, Ibid., 23, 923, 1002, 1160 (1931).
(3) Hixson and Luedeke, Ibid., 29, 927 (1937). (4) Hixson and Wilkens, I b i d . , 25, 1196 (1933). ( 5 ) Pearce and Eversole, J . Phys. ('him., 28, 245 (1924). KECEIVnD
June 9 , 1047.
n Concentrate from onseed Hulls If. E. SHIVER Converse College, S p a r t a n b u r g , S.
c. Owing to the rapidly increasing industrial importance of furan chemistry, a study has been made of the possibility of enhancing the value of the cottonseed hull as raw material for the manufacture of furfural. To the initial advantage of highest pentosan content of a n y known plant material must be added the fact t h a t the unique structure of the hull permits concentration of the pentosan content by the simple, inexpensive expedient of milling and screening. By properly regulated forces of disintegration and by- fractionation a t t h e proper screening level, the raw material may be separated into two substantially lint-free fractions of approximately equal bulk; t h e coarser is more t h a n 50qc richer in pentosan t h a n t h e original material, and the finer is significantly improved in f a t and protein content. Thus, more t h a n 8Oq0 of the pentosan is separated and recovered i n little more than 50qo of the original bulk of raw material, and the remaining lint-free fraction is niore t h a n 659!0 richer in protein and approximately 100% richer in fat content. The lint removed €rom the niilled product is relatiiely clean and suitable for use in the cellulosic industries where short fiber length is unimportant.
HE chief sources of pentosans are the an-
COURTESY JOHN W l L E Y & SONS, I N C
Figure 1. Cross Section (nboce) and Elements in Surface View (below) of a Cottonseed IIuIl ( X 1 4 Q ) , According to Winton and W'inton ( 1 2 )
nually renewable agricultural wastes and residues, such it- cottonseed hulls, oat hulls, peanut hulls, coincobs, bagasse, flax shives, ctc. These materials possess the economic advantage of accuniulatiny a t central processing points and therefore bear no separate cost of gathering and transporting. Among these wastes and residues, cottonseed hull bran has the additional advantage of containing a greater per centage of pentosan than that of any knonn plant
May 1949
INDUSTRIAL A N D E N G INEERING CHEMISTRY
919
material (9). As it comes from the gin, cottonseed is composed of the kernel and hull, or outer shell, to which more or less fiber is still attached in the form of fuzz. The kernels are separated and converted into oil and meal; the shells with attached fiber, left as residue, are known as cottonseed hulls. Some oil mills remove the major portion of this fiber for sale as linters, and the remaining product is known to the trade as cottonseed hull bran. The pentosan content of cottonseed hulls and hull bran is variously reported as ranging from about 20% to more than 35010, the wide diversity being due in part to confusion in terminology. Some variation exists as a function of seed type and growth environment, but in general the lower figures refer to the hulls and the higher to hull bran. Both of these materials find use in the feedstuff industry, the former as roughage and the latter as filler for cottonseed meal and cake. The normal production of cottonseed hulls in this country exceeds a million tons per year and is still regarded as virtually waste material. One reason why agricultural wastes and residues, including cottonseed hulls, are so poorly regarded as potential raw material for industry is the cost of handling and transporting such light and bulky materials, and of chenlically processing materials of such low unit value. In view of the increasing importance of furan chemistry (6), any process that gives promise of simple and inexpensive concentration of the raw-material pentosan content a t the source should be of interest. Upon mild acid hydrolysis, pentosan is converted on an industrial scale almost quantitatively t o xylose (6) and thence to furfural. Although there is no present largescale use for xylose, its subsequent product, furfural, has found extensive industrial application (4). Indeed, the demand for furfural is expected to increase greatly in the immediate future as a result of its employment in place of coal tar chemicals in the manufacture of nylon ( I ) . STRUCTURE OF COTTONSEED HULL
Microscopic examination discloses that the hull of the cottonseed is a multilayer shell consisting of several well defined areas (Figure 1). TQan outer layer of dark-colored thick-walled cells the cotton hair or fiber is attached, ep, followed in turn by a layer of thin-walled cells with brown pigmentation, br, a shallow layer of colorless cells, w , a remarkable thick layer of palisade cells with reddish brown coloration, pal, and finally a composite, inner, brown pigmented layer of cells, a, b, c, in juxtaposition to the kernel or seed (18). The hull or shell is about 0.30 mm. thick; the central palisade layer of cells is about 0.15 mm. The unique structure of the cottonseed hull raises the possibility that the chemical components may be separately concentrated in the several layers of cells. I n an attempt to develop a n analytical procedure for separating the components of plant tissue without chemical change during isolation, some evidence of this fact was obtained (IO). However, anatomical studies attempting to fix the position of the pentosans of the cottonseed hull have been inconclusive because of the difficulty of distinguishing between pentosan and lignin with available microchemical tests where the two occur together as they do in this material (8). Confirmation of the fact that such concentration of pentosan content does exist in the hull of the cottonseed is provided by investigation of the so-called flue bran removed from these hulls. I n the process of delinting cottonseed hulls, the teeth of the revolving mechanism bite into the surface of the shell, remove a portion of the brown outer layers of cells along with the linters, and expose the surface of the reddish brown inner layer of palisade cells. Analysis of this flue bran, obtained as a by-product in the commerical linters beater from hulls of 35.2% pentosan content, reveals only 10.470 pentosan. I n view of the physical structure of the cottonseed hull, this analysis indicates that the pentosan content is concentrated in the palisade cell area in the ratio of not less than 3.5 to 1 of the remaining cell areas. Further confirmation of this fact is pro-
Milling and Collecting Unit Used in Large-Scale Trials vided by microscopic examination of the hull fragments constituting the two fractions obtained in the milled and screened material to be described. Pentosan was determined in the usual manner by conversion to furfural and precipitation of the latter as the phloroglucide ( 2 ) . Protein, f a t and carbohydrate viere determined by the conventional methods of analysis of stock feeds ( 5 ) . Fractionation of the milled hull material was determined by means of the 15-minute Tyler Rotap screening procedure. PILOT-SCALE MILLING
The structure of the cottonseed hull indicates that a milling schedule, designed to separate the surrounding pentosan-poor layers of cells from the central, inner layer of palisade cells, should involve disintegration by attrition rather than by direct pressure or impact, Attrition is essentially the action of rubbing which tends to wear off surfaces and leave a smooth residual core; thus the effect is achieved largely through longitudinal cleavage as opposed to cross-sectional break. If there is a sensible difference ifi density or hardness between surfaces and core, as is true of the cottonseed hull, the subsequent disintegration results in a product that is readily separated into fractions, the particles of which are distinctly different in density and diameter. The relative proportion of the pentosan-rich palisade cells t o the surrounding cells is not greatly differentfrom 1to 1by volume (Figure 1); the pentosan content of the former, as revealed by flue bran analysis, is in the ratio of not less than 3.5 to 1 of the latter. Hence, the possibility exists that by efficient separation of the cell layers, more than 75 to 80% of the original pentosan content may be concentrated and recovered in little more than 50% of the original bulk.
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
980 TABLE I.
R E L I T I O N BETWEEN PLATE
OPENING
AND PARTICLE
SIZE
Screen >If,htl
_---------Plate 0,002
Opening, Inch-------0.01
0 , Ot5
0.075
~.-
0.10
Pentosan Content of Cottonseed I-Iulls
f 14
+20 -t 28 f35 6'! T6d - 65
-k
Lint
0.107~
...
0.50 11.00 23.45 17.30 38.70 8.92
...
...
0:9Z./, 14.10 24.00 16.14 36.13 8.81
2:61% 19.69 25.10 14.11) 29.81 8.60
... 6:3l% 26.63 22.00 10,29 24.30 10.47
Oo .lJ' 0.10 7.22 25.28 22.10 10.79 22.31 12.10
Pentosan Content of Cottonseed Hull Bran-
t 14
f20 f28 +35
+so
f 66 -65
...
... ... .. .. .. I . .
...
...
...
0.10
2.4G ...
2. ..9.3
0 3 . 91 60
17.2~
22,27 27.80 13.27 33.70
24.00 27.06 12.68 32.10
24.90 17.15 38.30
0,lO
:; 25.55 25.40 12.57 30.78
Vol. 41, No. 5
that the desired ratio by weight. a t the 60-mesh level is obtained with a minimum plate opening of 0.01 inch or less; at 50-mesh the optimum opening is 0.02 to 0.04 inch. I n view of increased energy consumption with diminishing plate opening, it is desirable to operate with openings as wide as possible; hence for both hulls and hull bran, milling with plate settings in the range of 0.02 t,o 0.03 inch and fractjionating a t or slightly above the 50mesh level seems best. The rapidity with which the curves of Figure 2 tend to become asymptotic beyond the 0.075-inch setting indicates that the practical limit of this varia,ble is near that point for these materials. Kext, i t was desirable to learn if t h e limit's of plate opening and screening level, within which the best ratio of hull and bull bran fractions may be secured, correspond with maximum 00x1centration of pentosan content in the coarser portion. The substantially lint-free fractions obtained by separation a t the 50-mesh level (Table I ) were analyzed for Dentosan cont,ent. The results are iisted in Table 11. For both materials t,he pentosan content, of the fract,ion QII L50-nieshscreen increased from 0.01- t o 0.05-inch plate selling, then decreased t o the minimum at the 0.10-inch opening. On the other hand, the pentosan content of the fraction through 50-mesh progressively decreased from 0.01- t o 0.10-inch setting; it, is noteworthy tha,t t,he lat'ter values are nearly the same for hulls and hull bran at the same plate opening. The fact that the fraction retained by 50-mesh screen is higher in pentosan content &I comparable plate sett.ings for hull bran as compared with hulls is significant. This is apparently due to increased entrapment of coarser hull fragments in the accoinpanyiny lint, and in less degree to the cushioning effect of the lint on extent of disintegration as plate settings are enlarged. Thus, percentages of both lint, and pentosan content, progressively ixicrea,se with increase in plate opening. The sinall quantity of lint associat'ed wilh hull bran has no appreciable effect on the pentosan content, of the material ret>ainedat the 50-medi level. I
5 The relativelysinall proportion of lint (2 t o 2 . 5 7 , ) associated with cottonseed hull bran was n o t separated and thus is included in t,he figures for the various screen fractions.
For industrial purposes it is desira,ble t o dcterniirie n-hether this simultaneous reduction of bulk and concentration of pentosan content can be accomplislied on a large scale, and whether milling devices of high throughput characteristics and low energy demand can be employed. Of the commercial-scale milling devices whose action largely involves disint,egration through shearing action by attrition or abrasion, it appeared t,hat the modern high-speed attrition mill would meet the requirenients with greatest economy and oficicncy. T o determine the relatioi~between attrition mill plate opening a i d particle size of resulting product, a series of small-scale 12- to 15-pound samplcs of bot'li hulls and hull bran was milled with varying plate clearances. Ttic samples were disintegrated in a 3G-inch at,trition mill with plate openings ranging from 0.002 to 0.10 inch, and t,he product was aiialyzcti by the Rotag screen. The results (Table I ) indicate that the substantially lint-frec product can be separated into fractions of approximately equal hulk most effectivcly a t the 50-60 mesh screening level, depending upon the magnitude of the opening between milling plates. The ratio of palisade cells to surrounding cells of the cottonseed hull is approximately 1 to 1 by volume (Figure I ) ; hericc it would appear that separation of thc substantiaily lint-free product in this ratio should result in riiaximuiii concentraCion of pentosan iii the coarser fraction. Howcver, siiicc scrccn analysis is based on weigh 1 rat her til an volume, and sirice the ratio by weight (density) of palisade cclls to surrounding cells is of the order of 1.2 t o 1, it is of value to LL 0 plot the ratio of fractions on 50- and GO-niesh screens to thosc passing these levels in order to establish optimum plate openings and MILL PLATE OPENING (inches) scrrening Icvels. Figure 2. Ratio by Weight of Figure 2 shows Fractions on 50- and GI)-Mesh these ratios for the Screens to Unit Weight of Fractions Passing Each Screen various plate setCurves 1 and 2 represent substantially linttings and the two free milled hull and hull bran fractions, screening levels. respectively, separated a t the 60-mesh level; curves 3 and 4 represent the same, The data indicate respectively, separated a t 50-meeh level
" r - 7 7
TABLE11. EFFECT OF FRACTIOWINC AT 5 0 - i l . l ~LEVEL ~~ ON PENTOSAN CONTENT (DRYBASIS)
-
(Pentosan content of original delinted hull material, 35 . 2 % ) - Plate Opening, Inch--- -
Screening T,cvel
0.05
0.01
0 . 0'7.7
-
0.10
Pentosan Content of Cottonseed Hulls On 50-mesh Throiiah 50-mesh Lint (18-inesh)
44.4% 19.0 9.0
48.17, 13.5 '
10.1
43.4% 14.2 10.5
Pentoran Content of Cottonseed Hull Bran 45.0 49.2 51.3 On 80-mesh 13.7 Through 50-mesh 18 8 14 0
42.27, 10.3 12.fi
43 1 11 6
LARGE-SCALE MILLING
The small-scale results indicate that maximum pentosan content of cottonseed hulls and hull bran is concentrated by milling with plate opening in the range of 0.02 to 0.03 inch and separating the product a t or slightly above the 50-mesh screening level. At this point the ratio of fraction held on this mesh to that passing through is of tho order of 1.31 to 1 for hulls and 1.13 t o 1 for hull bran. T o learn if these relations are modified to any extent by milling on the large-scale basis, and to secure the requisite engineering data at this level of operation, approximately 100-pound batches of hulls and hull bran \
