Expression of Liquids from Fibrous Materials C. FRED GUR”AM1 AND HENRY J. MASSON New York University, New York, N . Y . T h e separation of solid-liquid mixtures by expression has been investigated theoretically and experimentally. When pressing is continued to equilibrium at a fixed pressure, the relation between pressure and sample volume is given by the equation d P / P = A d ( l / V ) ,whioh can be inteb / V . Evaluation of the constants grated to log P = a gives a formula for computing the volume of a batch of material under pressure, and, if the solids volume is known, the quantity of residual liquid can be determined by difference. Experiments verifying this relation are described, and certain limitations are pointed out. The effects of batch size and liquid characteristics are discussed briefly.
+
E
-
XPRESSION or pressing is a means of separating liquid and solid materials which has been used for thousands of years. Biblical literature contains frequent references to the use of presses for producing wine or oils, and many native tribes today press out vegetable oils by primitive methods which have been in use for centuries. Expression is a special case of filtration, but in modern chemical industry the principal application of filtration has been in the separation of relatively thin or pumpable slurries. For mixtures which are too thick to flow readily, ordinary filtration equipment and technique are not suitable, and it is necessary to resort t o pressing as a means of separation. This may be the case if the liquid is of high viscosity, if the liquid-solid ratio is low, or if the liquid phase is not continuous, &s in materials of cellular structure. The operation of .pressing consists essentially of enclosing the material in a suitable filtering or retaining envelope and applying external pressure under conditions such that the liquid is forced t o the outside, where it can be removed. This operation has received comparatively little attention in the chemical engineering literature. Many industries employ expression for separating liquid-solid mixtures, notably in milling sugar cane, manufacturing vegetable and fish oils, dewatering peat, paper pulp, and textile materials, and disposing of industrial wastes, garbage, and sewage sludges. Recent technical improvements in pressing equipment have led to the use of pressing operations for the removal of mother liquors from crystals and for the recovery of chemical preoipitates and 1
Present address, Whitney Blake Company, Hamden, Conn.
Compression
sludges comparatively free of liquid. Each of these industries has been developed practically independently of the others, and there has not been much exchange of ideas among the various fields. The present research was undertaken for the purpose of correlating data from the different industries with independent experiments, in order to set up general equations which could be applied to all materials-in other words, to develop expression as one of the chemical engineering unit operations. There has been some confusion in the literature between the operations of expression and extraction. The latter word has been used loosely to designate either operation, but it is clearer to limit the term “extraction” to the process of solvent extraction. The combination of expression and extraction into a single operation occurs frequently, however; for example, the milling of sugar cane involves a series of pressings with simultaneous or alternate applications of weak liquors, which act as solvents or diluents for the cane juice. I n certain cases either expresslon or extraction can be used to separate a liquid-solid mixture. For example, vegetable oils can be recovered by either operation; the extraction produces a fuller recovery and a drier cake, but expression usually gives a purer product free from other dissolved materials. The choice of a process depends, in any given case, on the purities desired in the products as well as on the economics of the recovery. PREVIOUS WORK
The writers have not been able to discover any published information on the operation of expression, except in relation to a particular industry or material. Apparently there has been no attempt to ctevelop fundamental theories of the operation. The most valuable previous study in this field is that of Deerr (d-S), who limited his tests to sugar cane and bagrtsse but whose data can be correlated diTectly with the present work. More recently Koo and co-workers (6, 7, 8) experimented with various vegetable oils and derived empirical equations relating the yield of oil to pressure, temperature, pressing time, and moisture content. DEVELOPMENT OF THEORY
When pressure is applied to a liquid-solid system for the purpose of expressing a portion of the liquid, the changes that occur may be of some complexity. The following discussion is based in part on theory and in part on observations from industrial and
Bending and Slipping
Figure 1. Mechanisms of Void Space Reduction in Compresston
1309
Disintegration
1310
INDUSTRIAL AND ENGINEERING CHEMISTRY
academic research. Some of the hypotheses presented Tere not investigated experimentally but are given in this paper for the sake of complet,eness and because they have not been advanced previously in published form. There are two phases to be coilsidered in a study of expression: (a)the condition of a system a t equilibrium, in which there is no flow of liquid or other change with the passage of time! and ( b ) the condition of a system not a t equilibrium, in which a flow of liquid or a decrease in volume, or both, are occurring under the influence of the applied pressure. Only the equilibrium relations have been studied in the present investigation, since this information is required before any efiective study of rates can be initiated. CoxPREssIox WITHOUT EXPRESSIOX. When a dry or nearly dry fibrous or granular material is placed in a chamber and subjected to moderate pressure by means of a piston, there is a decrease in the total volume which it occupies. This reduction in volume is due to tvio phenomena which take place simultaneously : a compression of the solid material and a decrease in the volume of the voids or interstices in the material. The first is generally of minor importance in the range of pressures used in industrial processes. The diminution of void volume, however, is of greater importance and is caused by any or all of the following changes (Figure 1): ( a ) deformation of the solid matter, other than compression, such as bending or flattening of individual particles; ( b ) slipping of solid particles over one another; and (c) disintegration of solid particles. These changes take place during the period in which the pressure is being increased and perhaps continue for some time after the pressure has been held constant. Thereafter the system remains a t equilibrium unless it is further disturbed by external forces. The changes mentioned refer to a dry solid under moderate pressure. Even if a small amount of liquid were present, it would not necessarily be pressed out, provided that the void volume remaining after compression was still sufficient to contain the total liquid and that the liquid was properly distributed within the solid to utilize fully the void space. It is apparent', however, that the total void space must decrease under the influence of the applied pressure.
Figure 2.
Successive Steps in Compression of a Solid-Liquid System
COMPRESSION WITH EXPRESSIOX. If the applied pressure is increased, the void volume will continue to decrease until it becomes equal t o the volume of the liquid initially present, assuming favorable distribution (Figure 2). Further compression will reault in a compression of t h e liquid phase or, if flow channels are available, in the movement of a portion of the liquid to the outside of the enclosing chamber. This flow of liquid is referred to as expression. The relations governing the compression of a material with or without the expression of liquid are most effectively studied by experiment on each type of material Tvhich is of interest. At present there are too many unknown factors involved to derive a completely general expression of'these relations. Some of these
Vol. 38, No. 12
factors, which would be expected t,o affect the apparent density of a solid under compression, are (a)the true density of the solid, without voids (this is not a simple property, but depends frequently on the nature of any liquid present, since there is often an over-all volume change rvhen a solid is wetted); ( b ) the compressibility of the solid; (c) the size and shape of the solid particles; ( d ) the brittleness or friability of the solids, as affecting particle disintegration; and ( e ) the resistance to slipping of particles over one anot,her, including the lubricating effect of any liquid present. EQCILIBRIUM FOLLOWING PARTI.4L EXPRESSIOK.A h o t h e r consideration is the compression of a wetted solid beyond t h e point. a t which the free voids have disappeared, so that only a solid arid a liquid phase are present. The liquid phase, which may or may not be continuous, completely fills the interstices between the solid particles. If such a syst,em has been brought t o equilibrium under applied pressure, it may be assumed that all movement of liquid nithin the mass has ceased. This implies that there is no general pressure gradient on the liquid from one point to another; therefore, t'he liquid is essentially at atniospheric pressure everywhere because there is free access to t h e outside. Local concentrations of pressure n 4 l undoubtedly exist in the liquid due to surface tension and similar forces, but,, in t h e main, the mass of liquid lends no support to the applied pressure iyhich is therefore borne by the solid phase. It is assumed here t,hat flow channels are available for the movement of liquid, as any enclosed pockets or cells of liquid would of course be capable of bearing a portion of the load. If there is a further increase of pressure beyond this point, t'he added pressure will be borne momentarily by both the solid and the liquid and both will be compressed, so that a decrease in the total volume results. If the liquid,is free to escape, it will do so because of the pressure difference between the liquid and the outside. The removal of liquid from the system will relieve a part of the applied pressure, or, if the pressure is kept constant, the load will be shifted to the solid portion, and as a result the entire mass will be further compressed. I n either case some equilibrium will occur x-here the solid again bears all the pressure, the voids are filled with liquid under substantially atmospheric pressure, and a part of the liquid has been expressed. I n the absence of any reaction, either chemical or physical, between solid and liquid, the equilibrium conditions should be largely independent of the nature of the liquid. The viscosity, for example, should not have any effect, since the attainment of equilibrium implies that sufficient time has elapsed t o eliminate all pressure gradient on the liquid, no matter how great its visco4ity. Such a chemical and physical inertness is perhaps of more theoretical than practical importance. The concept has some value as a basis for comparison but, must not be expected to lead to results of great value. I n the present n-ork the compression of dry materials was studied as a basis for further studies n.it,h wetted materials. UNSTABLE OR FALSE EQUILIBRIUM. I t is common knowledge, in commercial practice, t h a t a solid-liquid system may be coinpressed under a definite pressure and held until flow ceases, and t,hat, if the pressure is then released and reapplied, a further yield of liquid is obtained, This is particularly true if the once-pressed material is ground or mixed before the second pressing. This phenomenon may he attributed a t least in part to afalse equilibrium caused by a bridging or arching action within the mass, which protects a portion of the liquid from the expressing force. I n any experiment, such conditions of false equilibrium are bound to occur as a result of conditions which are not susceptible to control. Thus, the manner of loading the charge into the conipression chamber may have an effect on the pressing characteristics, and supposedly identical tests will show variations in the pressure-density relations. This effect was noted in the data of the present experiments and is largely a statistical error which is unavoidable.
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
December, 1946
.
QUANTITATIVE DEVELOPMENT.I n most applications of the expression operation, the point of greatest interest is the yield of liquid which can be obtained from a given batch of material under various pressures. Practically all of the previous work has been aimed a t predicting this yield of liquid in terms of the other variables studied. The problem can, however, be simplified by a slightly different approach. The quantity of liquid which can be pressed out of a batch of material obviously depends on the quantity initially present. In most cases.this added variable can be eliminated by considering, not the quantity of liquid expressed, but the quantity of liquid remaining after expression. If the quantity of residual liquid can be predicted as a function of pressure, the quantity of expressed liquid is easily determined by difference from the initial content. It was found possible to study the liquid content of a pressed material on a volume basis. The compression of the actual solid substance is generally negligible in comparison with the diminution in void space; hence, in a system being pressed, the decrease in volume for a given pressure increment is equal to the volume of liquid expressed. Thus it is possible to measure the liquid content of a batch of material while it is under pressure by determining the total volume and deducting the solids volume, the lattsr being computed from known weight and density. The relation of the volume of a mass to the pressure applied to it was studied by Deerr (6) for sugar cane and bagasse. He developed equations of the f o r p V X Pn = k , where V = specific volume, P = applied pressure, and n and k 7 constants for these materials. However, this equation is not accurate over any considerable pressure range, and Deerr points out that the ,value of n should be expressed as a function of P . Because of these limitations and because the equation is entirely empirical, further studies seemed desirable. EXPONENTIAL EQUATION.Further study of the previous work, particularly that of Deerr and the present writers, indicated that a simpler relation can be derived if the volume is expressed in reciprocal form-that is, as bulk density or weight of solid per unit volume of the system. It also became apparent that increases in pressure should not be in absolute form, but rather as a fraction of the total pressure. The hypothesis is therefore presented that any increase in pressure, expressed as a fractional increase over the existing pressure, results in a proportional increase in the bulk density of the mass.
where
P
= pressure
A
= constant
AP = increase in pressure AD = increase in apparent density based on solid weight and total volume
The equation in this form is suitable only for small changes and is better written in differential form. Instead of density, we may retain the reciprocal volume term as more useful in the present work : -=
A d(l/V)
The hypothesis may be restated as follows: The change in pressure required to produce a unit change in apparent density is proportional to the existing pressure:
The equatidns can be integrated to give the following optional arrangements : log P = a +b/V
p t p =
wherea, b = constant
a’b’(l/V) a”e(bl”/V)
I
1311
PRESS PLATEN
I
DIAL INDICATOR
ii
I/ ll
I I/8-INCH TEST
CYLINDER
PRESS PLATEN Figure 3. Equipment Layout
It is apparent that, if this hypothesis has any validity, a plot of log P against l / V for any experiment should result in a straight line. The experimental portion of this study was devoted to obtaining data on P and V using various materials and conditions in order to confirm the hypothesis. EXPERIMENTAL WORK
The equipment used in this research included the well known Carver laboratory press, which needs no description here. The press was equipped with a manifold for two gages, reading from 0 to 5000 and from 0 t o 20,000 pounds total load. The 11/8-inch test cylinder supplied with this press was used as the pressure chamber for most of the tests, but in a few experiments the 3 l / 2 inch cage was used. The volume of the material under test was computed from t h e cross-sectional area of the cylinder and the height of the mass in the cylinder. The latter measurement was taken by a dial micrometer mounted on a special block inserted between the press platens (Figure 3). The experiments described here were conducted on synthetic mixtures of solids and liquids rather than on natural materials. The purpose of this technique was to secure greater variation in test conditions, as well as ease in control and reproducibility. Only fibrous solids were studied, as early attempts to use granular materials were unsatisfactory and most substances pressed commercially are fibrous in nature. The materials investigated include paper pulp, sawdust, woolen yarn, absorbent cotton, wool felt, and asbestos fiber. They were pressed dry and wetted with water and various mineral oils. The following test procedure was adopted after considerable experimentation as giving the most useful results: The desired quantity of solid was weighed out in air-drycondition. If a wet pressing was required, a large excess of liquid was added and allowed to soak for some time. The material was transferred t o the test cylinder, which was then closed with the header and piston and inserted between the platens of the press, with the dial
INDUSTRIAL AND ENGINEERING CHEMISTRY
1312
1.6
TABLE 1 SPECIFICV o L u m
AKD I N 1'/B-ISCH
Expt. 1,3.009
G.Air-Dry Wt. Pressure. Lb./Sq. In. 250 400
500 600
800 1,000 1,200 1,600 2,000 2,500 3,000 4,000 6,000 8,000 10,000 12,000 16,000
20,000
BULKDEXSITYOF DRYCowox CYLINDER
Expt. 2,5.487 G. Air-Dry W t ,
2.108 1.797 1.666 1.568 1.430 1.337 1.264 1.155 1.084 1.018 0.975 0.904 0.822 0.768 0.730
0,474 0.556
1.106 1.216 1.302 1,370
0.878
0.705
0.879 0.941 1.006 1,053 1.139 1.259 1.350 1.419
0.702 0.664 0.626
1.423 1.505 1.597
...
...
,..
...
0.600 0.638 0.700
0.748 0.792 0.866
0.923 0.982 1.026
0.794 0,741
...
0,442 0,541 0.587 0,635 0 702 0.759 0,807
2.402 1,904 1.735 1.574 1.432 1 320 1.249 1.132 1.071 1.010 0.965 0,900
/v.
Pressure, Lb./Sq. In. 260
400 500
600 800
0.635 0,698 0 754 0.801 0,884
0,776 0.744 0.723
1.384
0.824
... ...
1,000 1,200 1,600 2,000 2,500 3,000
v,
1/ [', g./cc.
1.762 1.538 1.437 1.377 1.286 1.216
0.568
1.825
0.650
1,552
1.178
1.111
4,000
1.056 1.002 0.964 0.911
6,000
0.838
8,000 10,000
0.789 0.745
12,000
0.701
20,000
...
16,000
...
0.696
0,548 0.644 0.679 0,720 0,777
0,778 0.822
1.473 1.389 1.288 1.226
0.851 0.900 0.947 0.998 1.037 1.098
1.189 1.114 1.065 1.024 0.995 0.951
0.841 0.898 0.939 0.977
1.193 1.267 1.342 1.426
0.878 0.845
1.139 1.183 1.234 1.302 1.394 1.456
0.726
...
0.810 0.768
0.718 0 687
. . I
...
0.816
1.005
1.052
Expt. 11, 8.797 G . Air-IIry M't. cc./g.
g./cc.
1.908 1.617 1.511 1.432 1.317 1.244
0,524 0.618 0.662 0.698 0.759 0.804 0.838 0.908 0,940 0,974 0,998 1.042 1,105 1.158 1.223 1 300 1.428 1.487
v,
1.194 1.101 1.064 1.026 1,002 0.959 0.905 0,863 0.818
0.769 0.700
0.672
CoiToN
Pressure Lb./Sq. i n . 250 400 500
600 800 1,000 1,200 1,600 2,000 2,600 3,000
4,000 6,000 8,000
10,000 12.000 16,000 20,000
cc./g.
IN
4.358G. Air-Dry TYt. V, I,']',
~ . / c c . cc./g.
g./cc.
2.743 0.364 2.882 0 , 3 4 7 2.284 0.438 2.294 0.436 2.105 0.475 2.113 0.473 1.973 0,507 2.015 0.496 1.781 0.562 1.839 0.544 1.637 0.611 1.700 0 588 1.541 0.649 1.579 0.633 1.429 0.700 1:265 0:790 1.305 0.766 1.187 1.133 1.043 0.929 0.851 0.803
0.842 1.226 0.816 1.158 0.864 1.072 0.933 0,932 1.072 0.872 1.146 0.827 1.209
0.882
0.958 1.076 1.174 1.245
E x p t . 19, 5,000G . Air-Dry Wt. I-, I/T.,'
-
cC./g.
g./cc.
E x p t . 20, 5.000G . Air-Dry _____ M-t, I', I/V, cC./g.
P vs. I / V
?===-
i i .
P vs.
I I
S
0.4
I
v I
I
l
I
j
I
j
2000 4000 GOO0 Pressure, Lb./Sq. In. Figure 4. Typical Cur\e Showing I'ressureVolume Relation
after several minutes vias recorded as the volume characteristic of that, pressure. The load vias then increased t o about 300 pounds and again maintained until a constant volume was obtained. This was continued to the maximum force of 20,000 pounds, with larger pressure increments a t the higher pressures.
l/V,
~ ~ / ~ - I CYLIKDER KcH
Expt. 18,
1
I
. . I
TABLE 111, SPECIFIC S'OLL-MEASD BULKDENSITYo r OII.-WET Expt. 17, 2.733G. Air-Dry W t . V, 1/V,
I
0,576
0,934 0.990 1,036 1.110 1.213 1.289 1.343
...
E x p t . 10,7.445 0 . Air-Dry U t . I-, l/l', cc./p. g./cc.
cc./y.
I
0.416 0.525
TABLE 11. SPECIFICVOLUMEA N D BULKDEKSITYOF ~ V A T E R WET COTTON IX I~/s-IscHCYLIXDER Exgt. 9, 5.200 G . Air-Dry W t .
,
ti
Expt. 3 , 10.136 G ___._. Air-Dry \ V t . 1
2.264 1.849 1.701 1.574 1.425 1.317 1,239 1.188 1.063 0.994 0,950
'3
Vol. 35, No. 12
g./Cc.
. . . . . . . . . . . . 11542 0:GkS 1:563 01640 1.480 0 676 1.503 0.666 1.360 0 . 7 3 5 1.388 0.720
. . . . . . . . . . . .
1.216 0.822 1.227 0 815 1.131 0.884 1.144 0.874 1.068 0.936 1.078 0.927 0:985 i:oi5 0:9& i:oi5 0.924 1.082 0.923 1,084 0.847 1,180 0.841 1,180 0.803 1.245 0.805 1.248
. . . . . . . . . . . .
0.764 1.308 0.801 1.249 0,747 1.338 0.750 1.332 0.714 1.402 0.746 1.340 0.716 1.396 0 . 7 2 3 1.384 0.678 1.476 0.701 1.426 0.698 1.432 0.701 1.426
micrometer and support in place. Pressure was then applied t o the material by means of the hand pump. A low pressure was maintained for several minutes to allow preliminary settling of t h e solid and expression of excess liquid. The force was then slowly increased t o about 250 pounds and maintained at that pressure, while readings were made of the volume of the batch at 1-minute intervals. The constant volume which \%-asattained
DISCUSSIOX OF RESULTS
The detailed data .of all experiments are too voluminous to be included in this paper, but sufficient data are given in Tables I t o VI to illustrate the discussion which follows. Figure 4 presents the typical form of the curves for P against '5 and I' against l / V ; the data arc the averages of all tests on absorbent cotton, both dry and wet. The tlieory developed earlier in this paper stated that the pressure increment required t o produce a unit increase in density is direct,ly proportional to the total applied pressure:
If the theory is correct, log P plotted against l,/V for any experiment should give a straight line. Such plots were prepared for the nineteen experiments reported in this paper (Figures 5 t o 9) and also for twenty-five earlier experiments by the present writers and twenty-seven experiments by Deerr ( 5 ) Tvhich are given in sufficient detail to make t,his possible. All of these curves are close to straight lines, with these few exceptions: The curves for water-saturated cotton (Figure 6) deviate from linearity a t high pressures. Since these breaks are abrupt, they are probably not due t o any error in the theory but rather to the occurrence of some new phenomenon which is not noticeable a t low pressures. A possible explanation of this is an actual disintegration of the fibers by a cutting of softer portions of the mass on harder parts. This phenomenon was described by Wriglit and Bennett (9) as occurring during the high pressure bailing of cotton a t gins. There is some deviation from theory in the case of watersaturated wool felt (Figure 8, experiment 2 5 ) . The reason for this is not obvious but may be connected y i t h the different nature of felted material in contrast to the loosely packed fibers of the other test materials. The deviations of Deerr's data from the present theory are slight and are only statistically irregular, with the following exceptions: Two of his curves, a t pressures from 1 to 60 pounds per square inch, are curved on this semilogarithmic plot. It must be concluded that the present theory fails at such extremely low pressures. Three other curves, made from experiments in a cylinder of l/a-inch diameter, show strong curvature. The present theory is not applicable t o these tests, presumably because of the large wall effect of such a small cylinder. With these
INDUSTRIAL AND ENGINEERING CHEMISTRY
December, 1946
1313
TABLE IV. SPECIFIC VOLUMEAND BULKDENSITY OF COTTON IN
Pressure Lb./Sq. d .
3‘/rINCH CAGE CYLINDER
No Wetting Liquid Expt. 7 60 Expt. 8, 100 G.Air-bry G. Air-Dry Wt. Wt. V 1/V 1/V, V, CC.;~.
g./d
co./g.
g./cc.
Wetting Liquid, Water
ExpL.15,60 Expt. 16, 100 G . Air-Dry G . Air-Dry Wt.
V
oc./g.
Wt.
1/V,
g./oc.
V,
oc./g.
1/V,
g./oc.
6
V
2 lS2 5
250 400 600
2.186 0.457 2,096 0.477 1.811 0.552 1.804 0.654 1.898 0.527 1.827 0.647 1.650 0.606 1.631 0.613 1.691 0.591 1.630 0.614 1.646 0.647 1.600 0.687
.c
800 1200 1600
1.565 0.639 1.604 0.665 1.476 0.677 1.417 .0.706 1.403 0.713 1,350 0.741 1.396 0.716 1.313 0.761 1.311 0,763 1.272 0.786 1.362 0.740 1.245 0.804
4 0.8 p?”
2000
1.242 0,805
3-
a
1.219 0.820 1.322 0.756 1.200 0.834
TABLEV. SPECIFICVOLUMEAND BULK DENSITYOF WOOL FELTS IN 1 * / 8 - CYLINDER 1~~~
0.4 200
2000 Pressure, Lb./Sq. In. Experiments on Dry Cotton (log P
Figure 5. 250 400 600
1.696 1.548 1.404
800 1,000 1,200
1.319 , i:iO4
1,600 2,000 3,000
1.137 1.089 1.014
4,000 6,000 8,000 12,000 16,000 20,000
0.590 0.646 0.712
1.324 1,201 1.108
0.755 0:833 0,902
0.758
1.052
0.960
o:Sii
0:9S2
i:Ois
0.880 0.918 0.987
0.943 0.917 0.881
1.061
0.959 0.884 0.839
1.042 1.131 1.192
0.882 0.851 0.846
1.160 1.176 1.182
0.807 0.767 0.742
1.240 1.303 1.347
0.840 0.834 0.828
1.190 1.199 1.207
1.091
1.136
.... .. ...
... ... ...
i:zie
o:iie
1.184
1.111 1.058 0.958 0.914 0.845 0.810 *0.770 0.752 0,744
20,000 VI.
1/Y)
0.845
0.900 0,945 1,043 1.094 1.183 1.234 1.299 1.331 1.344
exceptions Deerr’s data must be considered as lending considerable support to the present theory. EFFECTOF SAMPLE SIZE. The effect of varying sample size in a given cylinder may be observed in the curves of individual experiments (6’igures 5 and 6). Thus in Figure 5, experiments 1, 2, and 3 differ only in sample sise. It may be noted that, at low pressures, the smallest sample is the densest and the largest is the lightest. This is in accord with previous experience of the writers on a great variety of pressable materials. The explanation probably lies in the greater absorption of pressure by bridging and similar action and by wall effect in the larger samples. I n these tests the difference disappears in the range of 600 to 2000 pounds per square inch. At higher pressures results are too erratic for conclusions t o be drawn on the effect of sample size. Since only two sizes of equipment were used, no statement is made on the effect of this variable, and, for this same reason, data obtained on laboratory units should not be extended to larger equipment without further evaluation of this effect. EFFECTOF LIQUIDPHASE.The effect of quantity of liquid was not considered in these tests. I n all cases the solids were thoroughly saturated with liquid and a considerable excess of liquid was present, so that there were no air spaces in the material being expressed. Under these conditions the excess liquid was readily expressed, and it is a logical assumption that the quantity of initial liquid had no effect on the test. I n many commercial pressings the quantity of liquid is not sufficient t o fill completely the initial voids in the solid, so that air spaces are present. On compression some liquid is undoubtedly expelled before the air is completely removed, so that the P-V relations are not simple. However, the air is probably expelled at relatively low pressures, so that even this condition soon approaches the conditions employed in the present experiments. Possibly this retention of air is the explanation of the deviation from theory of certain of Deerr’s low pressure tests.
0.4 I 200
I 2000
20,OOo
Pressure, Lb./Sq. In. Figure 6. Experiments on Water-Wet Cotton (log P vs. 1 / V ) 1.6
,-
1
2000 20,OOO Pressure, Lb./Sq. In. Figure 7. Experiments on Oil-Wet Cotton (log P V I . 1/Y) 200
,
The nature of the liquid is of great importance. It was anticipated that there would be no difference in the P-V relations of a dry and a wet fiber if there were no a f f i i t y between the fiber and the liquid. On the other hand, if physical or chemical combinations occur, the effect might be marked. This is beat illustrated by the experiments on cotton-pressed dry, water-wet,
INDUSTRIAL AND ENGINEERING CHEMISTRY
1314
Vd. 33. No. 12
1.6
TABLE V I . SPECIFICVOLCMEAKD BULKDENSITY OF ASBC~TOS FIBERIK 1'/8-INCH C Y L I N D E R JTITH 20-GRAM A I R - D R Y W E I G H T Expt. 27,
N o Wetting Liquid Pressure, Lh./Sq. In.
c
, ,
0.8 I
I
I -
.........
/
-__
d
? 2.0 0 .-5 3 B 3 1!6 d
24, DRY 25. WATER 26; OIL
1/
v,
v,
1/J7,
cc./g.
g./cc.
cc./g.
g./cc.
1,000 1,200 1,600
0.793 0.767 0.725
1.262 1.304 1.379
0.775 0.741 0.695
1,290 1,350 1.438
2,000 3,000 4,000
0.693 0,644 0.608
1.443 1.552 1.644
0.663 0.617 0.584
1,509 1.622 1.713
6,000 8,000 12,000
0,555 0,533 0.504
1.802
1.876 1.986
0.540 0.513 0.485
1.852 1.950 2.0G3
16,000 20,000
0:485 0.471
2.063 2.123
0.466 0.452
2.147 2.211
, 0.4
I
200 800 2000 Pressure, Lb./Sq. In. # Figure 11. Comparison of Dry a n d WaterWet Cotton in 3 l / ~ I n c h Cage Cylinder
-
- 27, DRY _ _ _28,- WATER
/'
1.2 1 200
Each point is an average of 2 determinations
I
I
2000 20,000 Pressure, Lb./Sq. In. Figure 9. Experiments on Asbestos Fiber (log P cs. 1/V) 1.6
T-,
Expt. 28, T e t t i n g Liquid, Water
,
1
I
DRY, EXPT. 1,2,3 m WATER EXPT. 9 IO. II
1000 2000 10,000 20,000 Pressure, Lb./Sq. In. Figure 10. Comparison of Dry, Water-Wet, a n d Oil-Wet Cotton in ll/B-Inch Cylinder 200
Each point is a n average of 3 or 4 determinations
and oil-TYet. I t is knoxm that cotton has a large affinity for water but exhibits no reaction with oils. Figures 10 and 11 show t h a t water-wet cotton is denser than dry cotton a t pressures up to 1500-2000 pounds per square inch. Between 1.500 and 2000 pounds pressure the curves cross, and wet samples are less dense than dry (density computed on a x-ater-free basis), Possibly a t
this pressure the fibers have been compressed by an applied force equal to the drawing-together force of the cotton-water affinity, and a t higher pressures the presence of water retards rather than accelerates the compression. The rapid increase in density of m,ter-wet samples a t high pressure i.s probably caused by eiit,irely different effects, such as fiber disintegration, as noted. The effect of oil on cotton is quite different from that of Tvater. Figure 10 shows that' the density of the oil-wet samples is soinewhat less than t h a t of the dry material a t corresponding pressures. It appears that there is no affinity between the oil and the fiber; the oil merely occupies a certain amount of space and is not completely expressible. These observat,ions are duplicated in the case of the ~ r o o felt l samples which TTere tested dry, water-11-et, and oil-Tvet. In the case of asbestos the dry and water-n-et curves are parallel throughout and not far apart. S o reaction between asbestos and water was anticipated, and this is confirmed by the relation between the curves. I n earlier, unreported tests by the present writers it was noted that t,he presence of liquids such as nitrobenzene, aniline, and vegetable oil was Jvithout effect on paper pulp, whereas water and glycol gave results similar to those noted here. EFFECT OF SOLIDPHASE.Because of t,he limited number of solid materials investigated in this report, it is not possible to make any definite quantihtive statements as t o their effect on the P-V relation. However, all of the fibrous solids studied so far seem t o obey the exponential laiv formulated in this paper. It is conceivable that t,he specific volume of fibrous materials approaches the true or nonvoid specific volume of the fiber substance itself as the pressure is increased. Some support for sucah a hypothesis is given by the follon-ing calculations: The density of cotton fiber is given in the literature as 1.56 grams per cc. Xncreasing this by a factor of 0.000045 per atmosphere ( I ) , the density a t 20,000 pounds per square inch pressure would be 1.BB,
December, 1946
INDUSTRIAL AND ENGINEERING CHEMISTRY
which is comparable with the highest value, 1.60, observed in these experiments. Similar calculations for wool, for a normal specific weight of 1.30, indicate 1.38 a t 20,000 pounds compared with an observed maximum of 1.35, This hypothesis was not considered further in developing the relations given in this paper, because there are too many assumed values involved and because the equation which has been developed is sufficiently accurate without this further refinement.
1315
cylinder). At lower pressures small samples are denser than large samples. The effect of liquid on the ressure-volume relation depends on whether the fiber is affectei chemically or physically by the I relation is verv close to liauid. If there is no reaction the P - ' t h i t of dry fiber. If the fiber is hydrophilic the presence of water causes higher density a t low pressures, compared to dry samples. T h e effects of other variables were considered in .this report, but they are of lesser importance than those mentioned or else the data are not sufficiently detailed t o warrant further conclusions.
CONCLUSIONS
A logical first step in the development of expression theory is the formulation of a relation between the volume of a n expressible material and the pressure upon it. For the series of fibrous materials studied in this paper and in previous work by the present writers and others, the following relation is found valid for most cases : P = ab('/v) log P = a' b'lV
+
This equation gives a close fit to observed data for experiments on cotton, wool, paper pulp, felt, sawdust, asbestos, sugar cane, and bagasse. It is not valid a t low pressures (below about 50 pounds.per square inch) or in cylinders of small diameter (such a? Deerr's '/r-inch Cylinder) but seems generally applicable otherwise. * The effect of sample size is negligible a t pressures greater than 500 t o 1000 pounds per square inch (in the ll/s-inch diameter
LITERATURE CITED
(1) Bridgman, P. W.,personal communication, 1941. (2) Deerr, N.,"Cane Sugar", London, Winthrop Rodgers, Ltd., 1921. (3) Deerr, N.,Hawaiian Sugar Planters' Assoc., Bull. 22 (1908). (4) Ibid., 30 (1910); Arch. Suikerind., 19,21-60 (1911). (5) Deerr, N., Hawaiian Sugar Planters' Assoc., Bull. 38 (1912); Intern. Sugar J., 14,lS(1912-13). (6) Koo, E.C., IND.ENQ.CHEM.,34,342-5(1942). (7) Koo, E. C., J . Chem. Eng. China,4,15-20,207-11 (1937);5,4752,69-73(1938);7,1-4,23-5(1940); 8,l-10(1941). (8) Koo, E. C., and Chen, S. M., Ind. Research (China), 6, 9-14 (1937). (9) Wright, J. W., and Bennett, C. A., U. S. Bur. Agr. Chem. & Eng., Mimeo. Pub. ACE-67 (1940). SUBMITTPD in partial fulfillment of the requirements of the degree of doctor of engineering science, College of Engineering, New York University.
Improved Preparation of Guanidine Nitrate K. G. HERRING, L. E. TOOMBS, R. S. STUART, AND GEORGE F WRIGHT University of Toronto, Toronto, Ontario
A new process is described by which guanidine nitrate can be obtained in 92449% yield from lime nitrogen, ammonium nitrate, and urea. The latter compound is carried repeatedly through the process, together with excess ammonium nitrate and some dissolved guanidine nitrate, and serves to keep the reaction mixture fluid at the temperature (120" C.) at which the conversion occurs. This anhydrous reaction medium reduces by-product formation, reactor corrosion, and difficulty with wet ammonia. The comparatively low reaction temperature obviates the necessity for pressure equipment and reduces hazard in manipulation of ammonium nitrate containing organic material.
T
H E chemical literature records three main sources*of guanidine salts-namely, from dicyandiamide, from cyanamide solutions, and from cyanamide salts. The first of these, dicyandiamide, does not seem to have been used as a principal source of guanidine nitrate before 1908, when a German patent (22) of t h a t date described its fusion with ammonium nitrate to produce the guanidine salt. This was generalized in 1912 (1) and then extended by Werner and Bell in 1915 (27) and later in 1920 (28). These workers, using ammonium thiocyanate, considered that, since dicyandiamide is converted a t 205" C. to cyanamide, this depolymerization constitutes the first step of the reaction in which cyanamide is subsequently ammoniated to guanidine. Davis (6-9) described and patented a similar anhydrous fusion process, which was also reported by Ewan and Young (II), Blair and Braham (S),and Kat6 et al. (16). The process works poorly below 160" C.; above 170" it proceeds exothermically but not explosively. Infusible by-products were always reported, to-
gether with guanidine nitrate. This work, especially t h a t of Davis, was reviewed in 1931 ($0). The Davis mechanism involves initial reaction t o form biguanide (the main product below 160' C.), which then would ammonolyze t o form guanidine nitrate.
/"
N
H
/H
N
H
NHlNOI
N
$
/" 160" C.
One may suspect, after carrying out this fusion in the laboratory, that it would be somewhat hazardous on a large scale. FurthermoPe, Davis (6)implies that the 80% yield obtained under anhydrous conditions can be maintained if water is added, but Blair and Braham (8)report a more modest 54% yield of guanidine nitrate together with large amounts of biguanide nitrate and unchanged dicyandiamide, when the latter substance is treated with aqueous ammonium nitrate at 170-180" C. under 50pound pressure for 3 hours. Our results are in exact agreement with those of Blair and Braham. Thus, while water or ethanol always seem to lead to incomplete conversion and to by-product formation, the use of a n anhydrous diluent-liquid ammonia-has been used successfully (14) by the American Cyanamid Company to give a clean product in good yield by a method relatively free from the danger inherent in the
