Solid to Solid Surface Extraction. Application to Removal of Impurities from Sheep Fleeces George Harcourt Robertson* and James P. Morgan Western Regional Research Laboratory, Agricultural Research Service, U. S.Department of Agriculture, Berkeley, California 94 770
Solid to solid surface extraction theory was developed in general for removal of surface impurities from soiled fibers by sorbent particles to illustrate process potential. T h e technique of solid-solid extraction was applied to removal of natural contaminants from the surface of wool fibers by a sorbent powder of expanded-bead polystyrene ( e b p s ) . Equilibrium constants were experimentally determined to b e 4.9 for solid to solid extraction of ether solubles (crude lanolin), 4.3 for extraction of water solubles (potassium Salts), and 4.8 for extraction of ash (dirt and potassium salts). Mathematical analysis utilizing equilibrium data showed that acceptably clean wool (less than or equal to 1.0% ether solubles) could be produced using two countercurrent equilibrium stages each of which consist of mixing by tumbling. six hot compressions, and an opening to separate t h e solid phases. A dirty-wool to powder mass ratio of from 1.3:l to 4.9:l would be utilized. Sorption selection test results showed e b p s to be more sorbent than alternate sorbents such a s kaolin, pulverized popcorn, and expanded perlite. Preparation technique greatly affected the sorbency of e b p s .
Introduction Mass extraction processes effect the transfer of a component(s) of interest from one phase to a second phase by virtue of phase preference. Extraction of components from the interior or surface of a solid phase may be achieved by contacting the solid-phase with liquid-phase extractants. However, situations can arise in which liquid-phase extractants are undesirable because of deleterious effects on the solid, absorption into the solid, corrosivity, poor stability, toxicity, necessity for treatment of the liquid to recover contaminants, or cost. In these cases alternatives to liquid extractants must be found. One alternative to the use of liquid extracting phases for the removal of surface impurities from a solid phase is the use of a second solid phase as the extracting phase. Extraction of impurity components from the solid base (raffinate) to the extracting solid by solid to solid extraction occurs in part because of selective sorption, which depends ultimately on selective molecular interactions between the component of interest and the two solid phases. Extraction into a solid phase can also depend on the extent of the subdivision of the phase since this affects the availability of the molecular groups. Finally, extraction into a solid phase can depend on the physical structure of the solid; i . e . , a solid extractant made up of tiny spongelike particles can extract liquid components by capillarity and by suction when alternatively compressed against and released from the surface to be extracted. Liquid so extracted may be contained in the wells or pockets of the extracting solid in greater amounts than predicted or expected on the basis of molecular interaction alone. This paper introduces a mathematical framework for the solid to solid extraction process. Emphasis is placed on describing the equilibrium relation to aid in the selection of candidate extractants. Process design schemes are indicated. Additionally, the paper describes application of solidsolid surface extraction to the classical problem of cleaning wool. This application was motivated by the need for an extractive process to produce wool a t a satisfactory level of cleanliness without the simultaneous production of large volumes of noxious and difficult to treat waste. Production of clean wool requires separation of the fiber from the surface-adhering ether-soluble contaminants (crude lanolin), water soluble contaminants (known in the 12
Ind. Eng. Chern., Process Des. Develop., Vol. 14, No. 1, 1975
industry as “suint” and composed of perspiration and fecal solubles) and miscellaneous components such as sand, dirt, and vegetable matter. In combination these contaminants make up as much as one-half of the fleece mass. They are commonly removed by a liquid extraction into a warm detergent solution. The resulting highly contaminated solution constitutes the major pollution load from wool wet-processing operations (Porter, et al., 1972). Liquid extractive cleaning (scouring) techniques have been proposed as alternatives to the commercial detergent or emulsion scouring method. These extraction methods utilize hydrocarbon solvents or water and hydrocarbon solvent mixtures, but have not been universally applied because of the failure to remove water solubles from the fleece and the excessive size of the equipment required. Other approaches which have been proposed include the “frosted” process in which the lanolin is embrittled by freezing and then shaken from the fiber, and the dryabrasive process in which contaminants are abraded from the surface with cleaning powders.
Solid to Solid Extraction Equilibrium. Extractive separations of a substance from one phase to another are limited by the equilibrium for the substance between the phases. This equilibrium is expressed by the equation
1’ = Ju(3i)
(1)
where y and x are the impurity concentrations (mass impurity per mass of impurity-free solid substrate) on the extracting and extracted or raffinate phases, respectively. In this form the relation expresses equilibrium between homogeneous phases. However, the application described herein is to an extraction in which the phases are not homogeneous and enrichment from one phase to another may be effected because of affinity for the substance of the extracting phase or because of the extent of the surface of the extracting phase. The consideration of the surface extent is not a consideration in extraction into liquid phases where interaction is available on a molecular level. Therefore, it is useful to factor the concentrations into a term c,, the surface concentration, which accounts for specific interactions between extractable matter and the associated phase (impurity mass/length*); a term s I , the surface to volume ratio, which accounts for the effect of
geometry on the extent of the surface of the extracting phases (length-1); and a term pi accounting for the density of the phases (mass/length3). The product ciSi/pi is equal to the mass of impurity divided by the mass of impurity free’ substrate phase. Substitution of these factors into eq 1 yields
As will be illustrated below, eq 2 is useful because it indicates how improvements in extraction may be affected by the chemical nature and the physical form of the extracting phase. For the purpose of the discussion, it is assumed that (2) describes a straight line; then the equation becomes
(3) where k is a constant. Although the product of all three factors must exceed a value of 1 to indicate preferential extraction from phase “a” to phase “b,” individual factors may in practice be less than 1. The first or surface specificity factor will contribute toward favorable equilibrium if there is specific preferential sorption on the surface of “b” relative to the surface of “a” (cb greater than ca). The second or surface-to-volume factor will contribute toward favorable equilibrium if s b is greater than sa, and this ratio value may be calculated for special geometric cases. For instance, the extraction of surface impurities from spheres of radius ra, where sa equals 3/ra, to spheres of radius q,,where s b equals 3/q,, is
(4) so that favorable extraction will occur only for Q, less than ra. For the case of extraction of surface impurities from long cylinders or fibers of radius rat where sa is 2/ra, to spheres of radius Q,, the ratio becomes (5)
The contribution of the third term to equilibrium can be computed for the special case of density difference. This case is the extraction of impurity from a homogeneous solid to hollow spheres of a second solid, an idealized form of foamed or expanded solid. Here, the density of the extracting phase used in eq 3 is the effective density (Pb’) of the foamed solid as calculated by dividing the mass concentrated near the sphere surface by the total volume of the sphere. If the radius of the hollow portion of the sphere is a large fraction “ f ’ of the radius of the entire sphere, then the mass of the sphere is approximately (1 f ) ( ? b ) ( 4 T r b 2 ) ( P b ) , the volume 4 ~ ~ 3 / 3 and , the effective density 3(1 - f ) p b . Assuming that pb (the density of the solid portion of the sphere) is approximately equal to pa, the contribution to equilibrium becomes
pa Pb’
=A-
1 3 1-f
The conclusion from this discussion is that solids can be effective extractants and that their effectiveness depends on specific surface interactions, the extent of the product surface and the relative phase densities. In particular, finely divided solids, the particles of which are foamed or expanded, should be very effective extractants. However, eq 4 may be used for prediction of equilibrium in only a few strictly defined systems, and it does not account for
R,xg
R,xl
Rtxg
R,xntl
Rnxn
R,xn+l
R,XN
Figure 1. Multistage countercurrent extraction of impurity from phase R at concentration x to phase E at concentrationy.
the capillary or suction uptake which was indicated in the Introduction. Therefore, experimental data will often be required to establish numerical values for the equilibrium constant. Attainment of Equilibrium in Solid to Solid Extractions. Extraction processes in general are optimized when equilibrium conditions are reached. Attainment of equilibrium in a solid to solid surface extraction requires rapid transfer of impurities from surface to surface, complete surface to surface contact, and thorough mixing of the phases to achieve uniform distribution of the impurities. Since these requirements may not be met in practice, the equilibrium state achieved will be a function of the processing conditions applied and will be defined by these conditions. Application of solid to solid extraction as a fiber-cleaning method where the fiber is the raffinate requires special precautions to ensure attainment of equilibrium. Each extraction stage in the cascade should consist of the following unit operations: fleece opening, fiber-sorbent mixing, fiber-sorbent equilibration, and fiber-sorbent separation or reopening. Fleece opening removes some contaminants (vegetable matter, loose dirt, and some crude lanolin), separates the fibers, and exposes them to extractive contact. Mixing ensures contact. Equilibration consisting of repeated heating and compressing and remixing ensures more effective contact and facilitates transfer of impurities. Reopening removes sorbent and impurities from the fleece. Countercurrent Extraction. Since the values of the equilibrium constant are not always large, and since “real equilibrium” values are encountered which are less than the ideal equilibrium values, it is frequently necessary to expose the material bearing the component to be extracted to more than a single contact with the extracting phase. Generally, the most efficient way of making multiple contacts is countercurrently where component-rich substrate is contacted first with the component-rich extracting phase and finally with the component-free extracting phase. Efficiency results because the quantity of extractant required for a given separation is minimized. Countercurrent extraction is illustrated in Figure 1 where extraction stages (boxes) are numbered from the end which is fed highly contaminated raffinate (the solid phase bearing the components of interest). Raffinate and solid extracting phase flow countercurrently through the cascade at mass flow rates R and E (mass of clean, dry material). The concentration of impurity leaving stage n on the raffinate is x,, (mass of impurity/mass of clean, dry raffinate) and on the extract is y n (mass of impurity/mass of clean, dry extract). The variables in this problem are related by the equilibrium described in eq 7
and the operating line equation x,,+l
= ( R / E ) ( X ,-
.YO)
+
8)
Equation 7, which is predicted or experimentally measured, relates concentrations which are leaving the same stage in the cascade. Equation 8, which is the material Ind. Eng. Chem., Process Des. Develop., Vol. 14,
No. 1. 1975 13
Table I. Relative Sorption Capacity of Sorbents
Sorbent
Sand C o 1-11me a 1 Kaolin Expanded perlite Expanded st a r c 11 Expanded po 1y s t v r en e
Sorbed oil, oil/g of sorbent)
Particle size, mesh no.
Screen aperture, n i in
x 100
-18+35 -18+35 -45 + 60 -35 - 4 5
0.5-1.0 0.5-1.0 0.25- 0.35 0.35-0.5
epbs-H > ebps-R. Experimental Definition of an Equilibrium Contact. An equilibrium contacting stage as defined in the context of this discussion encompassed three major component parts. These were mixing or powdering, heating and compressing, and separating or depowdering. The effect of each component on the equilibrium (cleaning) achieved is shown in summary in Figure 4A, 4B, and 4C. (Also see Table 111.) Details of each action are discussed below. In each section of this figure column 1 represents the raw material-opened wool; column 2 represents opened wool 16
Ind. Eng. Chem., Process Des. Develop., Vol. 14, No. 1, 1975
~~
in:
((
((
lng, ( c
mg,
'C
~
Water solubles Ether solubles
13 11
0 35
Ash
43
11
71
16
52 22
3 22
which was powdered and depowdered; column 3 represents opened wool which was powdered, heated to 19O0F, and depowdered; column 4 represents opened wool which was powdered, compressed at room temperature for six 30-sec compressions to 20 psig, heated to 19O0F after the last compression and depowdered; column 5 represents opened wool which was powdered, compressed for six 30sec compressions to 20 psig and heated to 190°F simultaneously, and depowdered; and column 6 represents opened wool which had the treatment of column 5 except that a wet powder at 75% moisture was used. The fractional cleaning contributed by each of the actions of powdering, heating, and compressing was calculated by assuming that the contributions were additive and by dividing the additional amount of cleaning accomplished by the maximum cleaning observed. For instance, the contribution of simultaneous heating and compressing to the total cleaning achieved was computed by dividing the difference between columns 3 and 5 by the difference between columns 1 and 6. Table I11 shows the result of the calculation. The order of effectiveness of each action for impurity removal was different for the individual impurities which were assessed. For instance, the order of effectiveness for removal of water solubles was heating and compressing (71%), wet powder (16%), powdering (13%), and heating (0%). For ether solubles, the order was heating and compressing (52%), heating (35%), powdering (ll%),and wet powder (3%). Finally, for ash, the order of removal was powdering (43%), heating and compressing (22%), wet powder (22%), and heating (11%). Powdering was important to removal of all impurities and especially to the reduction of ash (an index of the dirt, silt, and miscellaneous matter). This occurred because the wool structure flexed during the tumbling action of powdering, and this mechanical action allowed loosely held inter-fiber impurities to be rejected. Reduction of water- and ether-soluble matter also occurred during this stage by adherence to the rejected dirt impurities, The importance of temperature to the effectiveness of the cleaning during compression was established previously (Robertson and Morgan, 1973), Temperatures above 150°F were found to be sufficient for effecting liquefaction
A
Table IV. Calculated Processing Ratio for Solid-Solid Extraction of Wool Desired concn of ether soluble matter, No. of index gk stages 0.25
0.50
Obtained collcn of
Clean Dirty mass mass ratioa ratio*
Bulk volume ratio'
water soluble matter
1
0.07
0.1
0.1
0. 51
2 3 4
0.65 1.35 1.95
1.3
0.61 0.68
1 2
0.15
1.3 2.8 3.9 0.3 2.0 3.6 0.6 2.9 4.9
2.7
3.8 0.3 1.9 3.5 0.6 2.8 4.7 0.9 3.6 5.6
t
0.94 1.20 1.75 1.30 2.20 0.32 1.00 1 1.41 2.40 2 2.35 2.60 3 1.0 3.20 0.49 1.50 1 1.81 2 3.7 3. 50 2.82 5.8 3.80 3 1.09 3.00 1 2.3 6.40 2.2 2.92 6.0 6.80 2 5.8 8.3 7.10 4.03 3 8.1 a Clean mass ratio is ratio of clean, dry wool to clean, dry powder. Dirty mass ratio is ratio of dirty wool t o clean dry powder. Bulk volume ratio is ratio of wool volume (2.8 lb/ft3 density) to powder volume (2.9 lb/ft3 density). 3
I
0.71 1.10
0
1
1 20
of the greasy material and for facilitating its transfer from the fiber to the powder. Temperatures of 190DFwere utilized in the present study, hence the high order of effectiveness of heating for reduction of ether-soluble matter (Table In). Temperature was not expected to be and was not observed to be of as great importance to the removal of water-soluble impurities since they remain in the solid form in the range of temperatures studied. The most important action for reducing water- and ether-soluble contamination was the simultaneous application of heat and pressure. This result emphasized the importance of obtaining good physical contact between raffinate and extractant under the appropriate conditions. The effect of the number of compressions and the pressure of compressions on the completeness of the equilibration was also assessed. Residuals of ether- and water-soluble matter were determined after equilibrations with moisture-free extractant at pressures in the range of 0 to 60 psig that were repated 1 to 5 times. Results are shown in Figure 5A and 5B. Fiber residuals were significantly reduced by repeating compressions up to a total of six at non-zero pressure up to 60 psig. Four compressions were also tested (though not indicated in the figure) and yielded residuals (4.4%water solubles and 1.95% ether solubles above 20 psig) which were slightly higher than obtained after six compressions (3.570 water solubles, 1.8% ether solubles). The improvements in impurity reduction caused by repetition of the compression resulted in part from the mixing applied to the sample before recompression. For example, each additional compression allowed for utilization of powder which had not been involved in previous contacts and for the dilution of the contaminants from heavily enriched powder particles to lightly enriched powder particles. Most of the reduction of residual impurity occurred during equilibiations a t pressures between 0 and 20 psig. Applied pressures above 20 psig showed only slight addi-
I
l 40
l
l
l
I
I
60
COMPRESSION PRESSURE (prig)
Figure 3. A. Ether soluble residuals after one (A), two (R), and six (C) compressions. B. Water soluble residuals after one ( A ) , two (B), and six ( C ) compressions. tional reduction (9%) of residuals during a single compression and no additional reduction after two, four, or six compressions. Consequently, 20 psig was selected for the data of summary Figure 4 and for use in equilibrium studies to be described below. The substitution of wet powder a t 75% moisture for dry powder was the second most effective action in reducing the water-soluble residual. This effectiveness occurred because soluble salts dissolved in the moisture and were thereby transferred to the powder. This action bears some analogy to the action of heat in liquifaction of the ethersoluble matter. As expected, the addition of water had little influence on the removal of ether-soluble matter. The presence of moisture in the powder was also beneficial because it eliminated electrostatic attraction between fibers and powder particles and between equipment surfaces and powder particles. Although powders a t this level of moisture tended to agglomerate slightly, they were still free flowing and distributed effectively into the wool structure during mixing. During equilibration, wool moisture increased from 5.3% to 12.670. The equilibrium contact described herein differs in several important ways from the study of the dry-scouring process described earlier (Robertson and Morgan, 1973). In the current work raw fleeces were opened according to conventional practice and then scoured by the dry or solid-solid extraction technique. This is a more difficult scouring task than was attempted previously when manually carded samples of fleece were dry scoured. For instance, the weight of fleece or raw wool (fibers plus all contaminants) from three to four fleeces (22 lb) was reduced about 23% in a single pass through the commercial opener utilized here. The material removed was largely greasy sand, vegetable matter, and a small amount of Ind. Eng. Chem., Process Des. Develop., Vol. 14, No. 1, 1975
17
short broken fibers. The “opened” wool produced from this operation was blended and was much more homogeneous than the raw fleece but did contain some clumps of fibers in essentially unopened condition as well as completely separated fibers. Fleece wool which was manually carded (10-20-g samples) was reduced in weight about 30% or more. This manually opened wool contained no fiber clumps or unseparated fibers. In this condition all of the fibers were accessible to contact with the extracting phase. However, no conventional processing equipment will open fleece wool to this degree. Another difference was in the equipment configurations utilized to apply pressure to the mixture. The earlier work utilized a single-drum drier equipped with application rolls to supply pressure; however, because of nonuniform sample density, pressure control was difficult. The present study utilized a ram or piston compression chamber which effectively controlled pressure for desired time periods while maintaining the temperature of the heated sample. Aside from being a useful research tool, this device could be a prototype of a full-scale commercial system in which the wool-powder mixture would be “pumped” through the scouring plant. Several differences between the current ‘and former work were concerned with the sorbent powders. Powders containing significant amounts of water (up to 75% moisture content) were more effective in removing water-soluble contamination than dry powders. Finally, the powder utilized in this study was the highly sorbent wet-milled (ebps-I) powder, whereas the powder utilized in the earlier study was the dry-rotary-cut (ebps-RC) powder with no added moisture. Solid-Solid Equilibrium. Solid-solid contacting of wool fibers was conducted to generate equilibrium data upon which to make an estimation of process capability. Data obtained by equilibrium contacting are shown in Figure 6A, 6B, and 6C. Conditions of equilibration were indicated as in column 5 and 6 of Figure 4. Equilibrium constants were 4.9 for extraction of ether solubles, 4.3 for extraction of water solubles, and 4.8 for extraction of ash. Thus, the solid-solid extraction described herein shows slight preferential attraction for removal of ether-soluble matter. This would be expected since the powder (polystyrene) bears more chemical resemblance to the ether-soluble constituents then to the water-soluble constituents. Utilizing these constants and the analysis described above (eq 9), several possible operating sequences were computed for wool having 16.5% ether-soluble matter and 31.6% water-soluble matter. The results of the analysis (Table IV) indicate that to obtain wool with 0.25% ethersoluble matter, 1.0 lb of powder will be required to clean 0.1 lb of dirty wool in one stage, 1.3 lb of dirty wool in two countercurrent stages, 2.7 lb in three countercurrent stages, and 3.8 lb in four stages. If a 3% concentration can be tolerated, then 1.0 lb of powder will scour 2.2, 5.8, or 8.1 lb of wool in one, two, or three stages. Intermediate requirements are indicated in the table. The operating conditions of greatest interest are those in which the dirty wool volume to powder volume ratio exceeds 1. These conditions result in the concentration of the impurity into a smaller volume with attendant advantage for subsequent powder regeneration. The result of applying the stage number and R / E ratios determined for ether-soluble equilibrium conditions to the calculation of the expected concentration of water-soluble matter yields a residual which is about 2 to 2.5 times the ether-soluble residual (see Table IV). This results because 18
Ind. Eng. Chem., Process Des. Develop., Vol. 14, No. 1, 1975
A
FIBER PHASE ETHER SOLUBLES (INDEX X 100) B
I / / / /
I
0
I
10
I
30
20
FIBER PHASE WATER SOLUBLES (INDEX X 100)
60
L
.. 20
2
I/
I
k’4.8
I
0 0
10
I
I 20
I
I 30
FIBER PHASE ASH (INDEX X 100)
Figure 6. A. Equilibrium of ether-soluble impurity between powder and wool. B. Equilibrium of water-soluble impurity between powder and wool. C. Equilibrium of ash impurity between powder and wool. the water-soluble equilibrium is less favorable than the ether equilibrium, and in the example shown, the watersoluble contamination is greater than the ether-soluble contamination. If this residual is too high, the design can be altered to set stage-number and R / E ratios based on the allowable water-soluble residual, and then the expected ether values can be determined. However, if ether residual requirements are stringent, the solids-extracted wool could be given a mild rinsing in water to reduce the water solubles while maintaining the ether-soluble residual at the desired level. As indicated above, conventional practice normally dictates that the product wool from the scouring operation have a crude lanolin index of 0.25 to 3.0% for adequate carding performance (the exact value depending on the intended use). Preliminary experience with solids-extracted wool indicates that slightly higher levels of crude lanolin may be tolerated since these occur as a result of the
lack of homogeneity of the raw prodiirr and arc reduced during c nrding For inmnre, during a sample automarir machine carding, levels of ether solubles in a solid-extracted sample of wool were reduced from 2.1 to 1.6%. This result indicates that higher levels of contaminants than oh1.ained from aqueous extracted wool may he tolerated if t he cleaning action of the carding machine can he utilized. However, this aspect of the cleaning and the effect of the level and types of impurities on long-term carding Iierformance remain to he examined. Reduc,tion to Practice. The data ohtaint?d above indicate thait the solids extraction process can p roduce a suffi.,.. ciently clean fiber provided a suitable numDer 01r equiiiorium contacts are made at the proper ratio of flows of extractant and raffinate. However, reduction of this method to practice will require evaluation of factors which are of as great importance to the wool industry as the fiber cleanliness. One of these factors is entanglement of the fihers and the resulting effect on the average fiber length. As practiced above, the solids-extracted wool was suhjected to tumbling and openings which were respectively more vigorous and numerous than those normally applied to wool. The solid-solid extraction procedure required three openings and a period of tumbling for each stage. Emulsion scouring requires only one opening, hut the opened wool is immersed in warm water where agitation can lead to entanglement. The result of the entanglement which occurs is indicated qualitatively in Figure 7 which compares the carded product of an emulsion scouring to I~ ~
~
the rslrrlnrl -.TALIaLCL-IIT .+---+&.-.y.ycu nr,4..r+ rl"y"" nf ".,...t***" "*os= onl:d Y"ll" " . . I
"I
1.
. ..
I"-..DC"",.
ing. Raw material for this comparison was drawn from the same sample of opened wool. Entanglements appear as tiny white knots in the carded product (sliver). Reduction of this entanglement is a goal of a more refined solids treatment which utilizes more sophisticated and less physically damaging opening devices and powder mixing devices. Cost of the process is an additional factor of importance to the acceptability of the technique. One negative cost factor is the cost of fully prepared extracting powder. This factor is dependent on the supply of styrene monomer. Another factor is the cost of powder regeneration. The regeneration of the powder to recover by-products and to extend the powder lifetime is essential for utilization of the technique. Methanol was used during regeneration studies quantitative recovery of by-products and with no detectable degradation of the powder particles. The product of this extraction was a viscous brown liquid which was reextracted with hexane to recover the lanolin and leave a residue which could he calcined for its potassium value. However, alternate solvents to methanol may prove to he of greater applicability with substitute extracting powders that have wider solvent resistence than is available in expanded polystyrene. The size of solvent regeneration equipment required for power processing should he small relative to the size of the equipment required for direct solvent scouring of wool. The relative size can he estimated by assuming dirty wool is solvent scoured a t a hulk volume of 2.8 lh/ft3 (Saville, et al., 1971). Then the powder regeneration extractor will need to process a total volume which is a fraction (Table IV) of the total volume of wool. For example, for two solids extraction stages the fraction varies from 0.34 for a 1% ether soluble residual to 0.77 for a 0.25% ether soluble residual. Positive cost factors include the income from the recovered by-products, Both the potassium salts and extracted lanolin have many established uses (Truter, 1956). Their value will depend on their chemical quality (acidity of the
..
. . .. .. *. :
..
.N..
.-
.
..F
,.
Figure 7. Card sliver of emulsion scoured wool and solid to solid extracted wool.
crude lanolin) and on the demand for them in the market. An additional positive cast factor could be the value of the extracting solid after its usefulness as an extractant has been exhausted due to physical or thermal degradation. This value will depend on supplies of and demand for the raw monomer and the availability of recycle techniques to convert the polystyrene hack to the monomer. Other factors which will affect acceptability are color, the hand or feel of the product, and fiber mechanical strength properties. Establishment of the balance of all these factors will only result from a controlled comparison of wool cleaned by the emulsion method and by the solid to solid extraction method. Cone lusions
1. :Mathematical analysis of solid-solid equilibria indicates that favorable extractions can be achieved and that exparided or foamed materials should be effective extractants 2. 1Powdered forms of expanded-head polystyrene were found to he more effective extractants (mass basis) than expanded starch, expanded perlite, and selected unexpanded materials. 3. Equilibrium constants in the expanded polystyrene powder-wool system were found to he 4.9 for ether-soluble matter, 4.3 for water-soluble matter, and 4.8 for ash. 4. Scouring by counter-current extraction concentrates the impurities onto a small volume of powder which can he subsequently processed by solvent extraction (methanol) for by-product recovery and powder reuse. 5. Several processing conditions will adequately scour wool; however, two or more stages at dirty-wool to cleanpowder ratios of 1.3:l to 8:l will encomoass most of the conditions required. Nomenclature a = extracted phase (raffinate) h = extractingphase cI = surface concentration on phase i (mass/length*) E = extract flow rate (masspime) EBPS = expanded-head polystyrene ehps = expanded-head polystyrene powder f = ratio of radius of inner surface to radlus of outer surface of a hollow sphere H = hammer-milled powder Ind. Eng. Chem., Process Des. Develop., VoI. 14. No. 1, 1975
19
I = incrementally-cut powder
k = equilibrium constant
n = index number for equilibrium stage N = total number of stages R = raffinate flow rate (mass/time) RC = rotary-cut powder ri = radius of particle of phase i s1 = surface to volume ratio (length-1) for particle of
phase i
x = ratio of impurity mass to raffinate mass xn = ratio of impurity mass to raffinate mass leaving stage n y = ratio of impurity mass to extract mass y n = ratio of impurity mass to extract mass leaving stage
n
p1'
= effeceive density including void volume of phase i (mass/length3)
Literature Cited Bennet, C.O., Meyers, J.E., "Momentum, Heat, and Mass Transfer," Chapter 39, McGraw-Hill, New York, N. Y.. 1962. Porter, J.J., Lyons, D.W., Nolan, W.F., Environ. Sci. Techno/., 6 , 38 (1972). Robertson, G.H., Morgan, J.P., Text. Chem. Coiorist, 5 (5),98 (1973). Saville, N., Shelton, W.J., Ward, R . . Sewell, J., West Riding Worsted & Woolen Mills Ltd., Comersai Mills, Cleckheaton, Yorkshire, England, 1971, Sherwood, T.K., Pigford. R.L.. "Absorption and Extraction," p 406, McGraw-Hill. New York, N. Y., 1952. Truter, E. V . , "Wool Wax,"Cleaver Hurne Press Ltd., London, 1956.
Greek Letters = density of phase i (mass/length3)
Receivedfor review November 5, 1973 Accepted July 18, 1974
pi
Use of a Centrifugal Pressure Nozzle as a Chemical Reactor Milton D. Marks, Jr., and Edwin J. Crosby* Department of Chemical Engineering, University of Wisconsm, Madison, Wisconsin 53706
The feasibility of using a centrifugal pressure nozzle as a chemical reactor for the manufacture of selected solid products in particulate form was demonstrated by the production of powdered sodium stearate. Stoichiometric quantities of molten stearic acid and aqueous sodium hydroxide were fed separately to the slightly modified adapter for the nozzle with the resulting mixture being sprayed into unheated air and collected as a flowable powder a short distance from point of atomization. Powders containing greater than 98 wt YO (dry basis) sodium stearate and ranging in size from 10 to 100 were produced.
Introduction
A major goal of process technology is the development of simpler and more efficient methods for producing desired products and performing required operations. In the specific case of particulate solids, a variety of methods are used. The production of many products may include a number of steps such as a chemical reaction, purification, precipitation, solvent separation, drying, and comminution. Such procedures can be costly in terms of the initial capital investment, operation, and low process efficiency. In addition, when numerous processing steps are involved, many variables usually must be controlled to ensure a product of adequate quality. The development of a simple and convenient means to transform the raw material directly into the particulate product with a minimum of intermediate processing steps offers definite advantages. For a process which involves the fairly rapid and essentially irreversible reaction of two or more liquid reagents to produce a nonvolatile product in the form of a molten solid, a slurry, or a concentrated solution, the intimate contact of these reagents in the body of a spray nozzle could lead to the direct production of particulate solids. If the reaction were completed within the nozzle proper, then any cooling and/or drying could occur in a spray chamber. If the reaction were not completed in the nozzle, it possibly could continue within the drops after atomization. Any necessity for further comminution of the solids could be entirely eliminated. However, the physical nature of solid particles produced in this manner might be different from that of powders produced by other meth20
Ind.
Eng. Chem., Process Des. Develop., Vol. 14, No. 1, 1975
ods. Further, the time required for processing could be significantly reduced. Finally, the number of variables which would have to be monitored to maintain a desired quality of product also might be reduced. In order to demonstrate the feasibility of this technique in the manufacture of an existing product, the production of a powdered soap was chosen for study. This product was selected primarily because the reagents are immiscible at low temperatures and thermal degradation can occur at elevated temperatures. System Studied The specific chemical system chosen to test the proposed processing scheme was the formation of sodium stearate by the direct neutralization of the fatty acid. This system was picked not only for convenience but also because of its commercial importance. Chemistry and Kinetics. Soaps can be prepared either by the saponification of a natural glyceride or by the direct neutralization of the fatty acids. The use of direct neutralization is a more recent development which was brought about by the advent of efficient processes for the continuous hydrolysis of glycerides. Sodium hydroxide and potassium hydroxide are the most commonly used alkalies for both saponification and neutralization. In the present study, stearic acid was reacted with aqueous sodium hydroxide. n-C1,H,,COOH
+
NaOH(aq) e n-C,,H,,COONa(aq)
+ HOH
