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Description  |
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SUMMARY OF THE INVENTION
This invention relates to the collection and storage of solar thermal
energy by means of viscosity stabilized solar ponds. In particular, the
invention relates to the use of a large body of fluid as the
collector-storage device. Water, clear oils, synthetic organic solvents,
glycols, anti-freeze compositions, and the like may serve as the
transparent fluid. The fluid must be clear, non volatile, stable to
sunlight, and readily available. A great many substances might serve;
however, good quality water is valued at less than 1/1,000th the cost of
the cheapest manufactured products, and it therefore seems likely that
only systems which are essentially aqueous will come into widespread use.
For this reason, the discussion below focuses on aqueous systems, but it
should be clearly understood that the principles, the control of the fluid
characteristics through the addition of selected solutes and additives,
apply in a very general sense to any liquids.
Normally, bodies of water or other low volatile fluids exposed in the sun
will gradually become slightly warm at the top and remain cool at the
bottom. However, previous investigation has shown that salts can be used
to stabilize a temperature inversion in an aqueous pond. H. Tabor and
others, have described the stabilization of large aqueous bodies through
the addition of salt at the bottom of the pond. See Tabor. H., "Solar
Collector Developments", Solar Energy, III, No. 3, pp 8-9, (1959), and
Tabor. H., "Solar Ponds", Electronics and Power, pp 296-299, Sept. 1964.
And Tabor has referred to solar heated ponds of this sort, stabilized by
means of a density gradient, and used to collect and store solar energy,
as solar ponds. Experimentation has shown that a properly designed density
gradient pond can support a substantial temperature difference--the order
of 100.degree. C. for ponds about one meter deep. See Weinberger, H., "The
Physics of the Solar Pond", Solar Energy, VIII, No. 4, pp 45-56, (1964).
A disadvantage of the foregoing arrangement is the tendency of diffusion to
undo the salt concentration gradient. A further factor which tends to
eliminate the needed concentration gradient is evaporation from the
surface of the pond. In practice, these problems can be managed by
continually adding fresh water at the top of the pond (washing the pond)
and supplying salt or freshly saturated salt solution at the bottom.
Material balance requires the removal of an intermediate layer in the
pond. This may be desirable in any case in connection with the extraction
of heat; but, a very delicate adjustment must be maintained in order to
insure stable operation.
A further disadvantage of the salt stabilized, density gradient solar pond,
is the very large amount of salt needed. Tests showed that sodium chloride
is only marginally useful; magnesium chloride works much better. On a
weight basis, a saturated solution of magnesium chloride contains about 53
parts of salt per 100 parts of water at 0.degree. C. and 73 parts of salt
per 100 parts of water at 100.degree. C. (sodium chloride 36/40 same
basis--less soluble and not so sensitive to temperature), and for a pond
one meter deep, the quantity of magnesium chloride required per square
foot of exposed (collector) surface is approximately 65 kg. (143 lbs.).
This salt is typically priced at about 13.cent./lb., and clearly, on the
basis of the large magnesium chloride inventory alone, the salt stabilized
pond is far from being the inexpensive large scale device originally
proposed.
It is an object of the present invention to avoid the foregoing
disadvantages of large solar ponds by using gelling agents or polymeric
thickeners to stabilize the fluid mass. Salts and/or other low molecular
weight solutes may be present in connection with requirements for
elevation of boiling point pH control, as fungicides, as corrosion
inhibiters, as ultraviolet absorbers, and so forth. However, the basic
concept of the present invention is stabilization through viscosity
control, or gelation of the fluid, rather than through the creation of a
substantial density gradient in the pond. In order for the thickened pool
to function as an efficient solar energy collector, the thickener and the
fluid used must be clear and transparent, and the presence of the
thickener must not appreciably alter the light transmission
characteristics of the fluid.
A further object of this invention is to provide a collector-storage system
suitable for heating a house or cluster of low-rise apartments. In this
usage, the dwelling or heated space will surround, or nearly surround, the
solar pond. At least 50%, and preferably more, of the pond perimeter will
share a common wall with the structure to be heated. In case the building
surrounds the pond entirely, suitable glazing--a south wall, or a
skylight, or both--must be provided. In the case of a smaller building,
the structure cannot really contain the pond but there are enormous
advantages to placing an air layer over the top of the pond. This could be
accomplished by floating a transparent plastic bag (quilt) on the top of
the pond. A similar device might also be used with interior ponds, but it
seems superfluous. The building envelope and the heated air within it
would serve essentially the same purpose. It is known that fluids confined
between walls spaced apart less than a critical distance will not support
convection currents; for example, air between sheets of glass spaced apart
approximately 1/4 of an inch or less will not support convection and thus
provides very good thermal insulation qualities. It is also known that
this critical dimension increases as viscosity increases.
It is therefore another object of this invention to prevent convection
currents in solar ponds through the use of dividing matrices having a
spacing of less than the critical dimensions. Other objects of the
invention will in part be obvious and will in part appear hereinafter.
The invention accordingly comprises several steps and the relation of one
or more of such steps with respect to each of the others thereof and the
features of construction, combinations of elements and arrangements of
parts which will be exemplified in the methods and constructions
hereinafter set forth. The scope of the invention is indicated in the
claims.
THE DRAWINGS
FIG. 1 is a perspective diagrammatic view of a viscosity stabilized solar
pond according to my invention.
FIG. 2 is a perspective diagrammatic view, a portion being broken away, of
a viscosity stabilized solar pond according to my invention.
FIG. 3 is a perspective diagrammatic view, a portion thereof being broken
away, of another viscosity stabilized solar pond according to my
invention.
FIG. 4 is a perspective diagrammatic view of another viscosity stabilized
solar pond according to my invention.
FIG. 5 is a perspective diagrammatic view of another solar pond according
to my invention.
FIG. 6 is a perspective diagrammatic view, a portion thereof being broken
away, of another solar pond according to my invention; and
FIG. 7 is a perspective diagrammatic view of another viscosity stabilized
solar pond according to my invention.
The same reference characters refer to the same elements throughout the
several views of the drawings.
DETAILED DESCRIPTION
FIG. 1 is a perspective view of a very simple version of the proposed
viscosity stabilized solar pond 12. Light falling on the surface 14 of the
thickened clear liquid 16 passes through the liquid layer and is absorbed
by the blackened bottom 18 of the pond. The absorption of light produces
heat at the bottom of the pond. The vertically hatched section 20 under
the black absorber in FIG. 1 is intended to indicate that a mechanism for
extracting some of this heat may be provided; however, for the moment, we
need not consider the details of this. With any normal fluid in the pond,
thermal expansion at the lower heated surface would produce a density
decrease at the bottom of the fluid mass, and ordinarily this density
decrease would drive convection currents which would tend to carry the
heat to the upper surface where it might be lost by radiation,
evaporation, conductive transfer to the air mass above, or all three.
Normally, it is impossible to maintain a situation in which the bottom of
a fluid layer is significantly warmer than the top. However, convection
currents cannot be established so long as the product of certain
characteristic quantities, the Grashof and the Prandtl numbers, remains
below a certain critical value. See E. R. G. Eckert and R. M. Drake, Heat
and Mass Transfer, 2nd ed., McGraw Hill, N.Y. (1959) p. 328. The Grasshof
number is given by:
##EQU1##
where: d = characteristic dimension, the depth of the fluid layer in the
case of a layer that has considerable lateral extent.
g = the acceleration of gravity
.DELTA.t = the temperature difference across the layer.
B = volume coefficient of thermal expansion of the fluid.
.nu. = the kinematic viscosity of the fluid (viscosity/density ratio). The
Prandtl number is given by:
Pr = .nu./.alpha.
where:
.alpha. = thermal diffusivity (thermal conductivity divided by specific
heat).
and .nu. has the same meaning as above.
The product, Gr .times. Pr, is sometimes known as the Rayleigh number.
##EQU2##
In a fluid layer that has both a top and a bottom cover, this quantity
must exceed about 1,700 before natural convection can begin. If the top is
not covered (called a "free surface"), the critical value is only about
1,100.
It is instructive to consider the maximum depth of water that is stable
against convection when the temperature gradient is 1.degree. C./cm. Using
the properties of water near 20.degree. C., and the relationship Gr
.times. Pr = 1,700, the maximum thickness without convection works out to
be about 0.6 cm. On the other hand, if we assume that a thickener has been
used to raise the viscosity to something like 0.66 .times. 10.sup.5 times
its normal value, and also using the properties of water at around
50.degree. C. (more appropriate to the solar collector-storage proposal),
then, for the same gradient, the limiting thickness (depth) for convective
circulation works out to be a little over 8 cm. An additional 1,000 fold
increase in the viscosity will raise this figure to 0.46 meters.
Materials and techniques which will allow us to increase the viscosities of
ordinary thin fluids to 10.sup.6 centipoises or more are available, and
some that are appropriate to thickening water are discussed further below.
However, such very high viscosities may present problems in terms of
materials handling, quantities of materials needed, the range of materials
that may be considered, and so forth. Therefore, in the interest of
allowing a proper engineering optimization, the slightly modified forms of
the viscosity stabilized pond shown in FIGS. 2, 3 and 4 are proposed.
The pond 22 illustrated in FIG. 2 is filled with a thickened fluid 24, has
a black-bottom surface 26 and a heat storage and/or heat transfer
mechanism 28 at the bottom just as the solar pond 12 in FIG. 1.
Additionally, it is provided with three transparent flat sheets 30, 32 and
34 which divide the pond into three layers each of thickness d. The added
sheets 30, 32 and 34 need not be perfectly clear; but under the conditions
of use, in contact with the thickened fluid, they must transmit a high
proportion of the incident light--preferably over 90% each sheet.
The intermediate sheets 30 and 32 serve primarily to control the onset of
convection and it is assumed that they will not greatly affect the
conduction of heat from the bottom of the tank to the top. The upper sheet
34 serves a dual purpose: it is part of the convection control system;
and, it retards evaporation. If it were not present, or if the evaporation
of fluid from the pool were to be controlled by floating a freely movable
non-volatile substance on the surface--a layer of high boiling clear oil
for example--then the spacing appropriate to the uppermost layer, the
depth of the top section of the pool, would have to be calculated using
the lower of the two critical Rayleigh numbers.
The control of evaporation is most important in terms of reducing heat loss
from the pond surface. However, there are other consequences of
evaporation that may also cause concern. The pool may, for example, be
operated in a closed space, inside a large house behind a suitably glazed
south wall, or under a skylight. Alternatively, one or more of the
ingredients of the working fluid might be particularly volatile; if this
were the case, evaporation would have to be retarded in order to maintain
the composition of the pool.
Control of evaporation does not require the use of an actual preformed
sheet in contact with the top surface of the pond. However, if a fluid or
movable upper layer--nonvolatile oil or a monolayer of cetyl alcohol for
example--is used, then as stated above, the depth of the topmost section
must be calculated on the basis of the "free surface" critical number.
In FIG. 2, the lower, vertically hatched portion 28 of the
collector-storage system is intended to suggest a mechanism for heat
transfer (extraction of energy) at the bottom of the viscosity stabilized
pond. It might seem feasible to place coils containing a suitable working
fluid in the lower, hottest part of the pond, but this would provide some
degree of shading and a preferred method for extracting the heat energy is
to make provision for the circulation of either air or water below the
absorber surface 24. The former might readily be accomplished by
supporting the bottom of the tank on hollow concrete blocks, or on a
suitably designed brick checker. A bed of rough stones might also be
satisfactory. The depth of brick, concrete, or stone would add to the
inherent storage capacity of the thermal gradient pond. Heat would be
withdrawn from the bed by circulating air through it.
If the thermal energy is to be extracted by the circulation of a liquid,
then the tank would simply be extended some suitable distance below the
absorbing sheet. The absorbing sheet would then become also a means for
dividing two fluids. The material in the under layer would not be
thickened, and it would be circulated as required to carry heat away from
the underside of the bottom sheet. Inexpensive storage could be added to
the normal capacity of the pond by increasing the depth of the under
layer. The physical arrangement is such that the warmest part of the
circulating auxiliary storage fluid will be the upper part of the under
layer, the portion that is in direct contact with the bottom of the
thickened collector-storage pond. Unit heat transfer rates are not
expected to present any severe limitations as long as we are considering
fairly thin sheets of material having considerable lateral extent.
Given any definite set of overall conditions: i.e., viscosity, temperature
difference, pond depth, coefficient of thermal expansion, density and
diffusivity, we can use
##EQU3##
to calculate a Rayleigh number for the system. If the calculated number is
above a critical value (1,700 or 1,100 depending on circumstances) we can
expect the pond to circulate; if the characteristic number is less than
the critical value, our expectation is that natural forces (buoyancy) will
be insufficient to drive convective circulation, and the pond should be
stable. Once a particular design objective has been established, and the
fluid properties have been fixed, there are really two approaches to using
the critical circulation conditions (Rayleigh number) to establish a
non-circulating configuration: a configuration such that the pond does not
circulate when heated from the bottom so that the bottom is warmer than
the top by a preplanned amount, .DELTA.t.
First, we could choose a total depth, d, shallow enough to prevent
circulation: i.e.,
##EQU4##
And this may be acceptable if .DELTA.t is to be rather small, 10.degree.
C. say, and if at the same time we can find ways to thicken the pool to
achieve a viscosity of about 10.sup.3 poise (10.sup.5 centipoise). Under
these circumstances, a depth of 11 cm or a little less would be
satisfactory for inhibiting natural convection. This situation is
illustrated in FIG. 1, and FIG. 2 is a variation on it. Perhaps we will be
unable to attain a viscosity as high as is specified above, or
alternatively, we may desire a .DELTA.t closer to 50.degree. C. rather
than only about 10.degree. C.; then to limit the circulation, we could
subdivide the total depth of the pond into a series of layers, each
calculated to have a depth "d" just less than the vertical spacing needed
to permit circulation. In performing this calculation, both the total
depth and the total .DELTA.t must be divided up into appropriate fractions
of the whole; and, the "d" and the ".DELTA.t" associated with each layer
must be used with the other appropriate fluid characteristics to establish
a non-convecting design. Actually, if the temperature gradient is linear,
we can use
##EQU5##
to establish the depth needed for each layer.
##EQU6##
A second approach to the use of the critical condition to limit circulation
is to calculate the R associated with any arbitrary design and then use
this R to design a critical lateral cell which is as small as, or
preferably smaller than the natural circulation cell that would be
associated with the calculated number. In doing this, we're really raising
the minimum buoyancy required for flow by introducing a planned amount of
viscous drag. That is, once the limiting lateral cell is in place, we can
expect the pond to be stable (non-convecting) as long as the Rayleigh
number used in designing the cell is not exceeded.
It may not always be convenient to limit the pond depth or to subdivide the
pond with horizontal sheets. There are other configurations which will
inhibit natural convection--see for example, FIGS. 3 through 7. When
convection first develops in a fluid layer heated from the bottom, a
cellular sort of flow pattern is set up. When the depth, temperature,
viscosity relationship is appropriate to the onset of convection, the
width of the natural cells tends to be the order of twice the depth as
calculated from the critical Rayleigh numbers mentioned above. Dividers
placed vertically in the fluid layer will inhibit circulation just as
effectively as the horizontal sheets which limit the thickness of the
layer. Given any set of fluid characteristics, a depth, and a temperature
difference, we can always calculate a characteristic Rayleigh number. And,
associated with each Rayleigh number equal to or above the critical
minimum there will be a small circulation cell. As the Rayleight numbers
increase, the lateral dimensions of the minimum cells associated with
circulation become smaller. By adding vertical barriers with a limited
lateral repeat distance, we can force the Rayleigh number appropriate to
circulation to be much higher than the value that would naturally prevail
for a fluid layer of indefinite lateral extent.
Referring to FIG. 3, a solar pond 36 containing a viscous fluid 38 has a
depth d and is divided into hexagonal cells by means of the hexagonal
matrix divider 40 which may be formed of suitable transparent material
such as plastic. Each face of the hexagonal cell has a lateral dimension
L. The pond is provided with the usual solar energy absorbing bottom
surface 42 and heat storage or a transfer mechanism 44.
The solar pond 46 illustrated in FIG. 4 similarly is provided with the
blackened bottom surface 48, heat storage or transfer device 50 and is
filled to a depth d with a thickened fluid 52. In pond 46, convection is
inhibited by dividing the pond up into elongated lateral cells by means of
vertical dividers 54 which are spaced apart a distance L. The dividers may
be of any suitable radiation transmitting material such as a plastic film
or the like.
In the solar pond 56 of FIG. 5, the viscous fluid 58 is contained with an
elongated plastic bag 60 of suitable radiation transmitting material. The
critical dimension in solar pond 56 is the dimension d of the bags, the
smallest dimension across them as will be explained below. The pond is
provided with a heat absorbing surface 62 and heat storage or heat
transfer device 64 as previously explained.
FIGS. 6 and 7 are illustrative of the concepts discussed below. In these
Figures, solar ponds 66 and 68 are filled with gelled fluid 67 and 69 and
divided into rectangular, vertical walled matrices as generally indicated
at 70 and 72 respectively. It is understood that these matrices are of
suitable radiation transmitting material such as plastic and are located
in a solar pond having a radiation absorbing lower surface 74 and 76
respectively and each storage or heat transferring devices 78 and 80.
Referring to FIG. 6, the square cell shown therein has a dimension L on
each side. Referring to FIG. 7, the rectangular cells shown therein has a
dimension L on the short side and a dimension r .times. L on the sides
perpendicular thereto.
The general problem of cellular circulation in a fluid layer heated from
the bottom has been very extensively studied. It turns out that the
characteristic lateral dimensions of the repeating cells are directly
proportional to the depth of the layer and inversely proportional to
dimensionless parameter "a" which increases as the Rayleigh number
increases. The table which follows relates the "a" Rayleigh number to the
dimensionless parameter for both the free surface (uncovered) and the
non-slip boundary (covered) cases. See Chandrasekhar, S., Hydrodynamic and
Hydromagnetic Stability, Oxford University Press, Oxford (1961). Chapter
2.
TABLE 1
______________________________________
CRITICAL PARAMETERS, Both Surfaces Covered
R "a"
______________________________________
1,707,8* 3.117
1,879.3 4.00
2,439.3 5.00
3,418.0 6.00
4,918.5 7.00
7,084.5 8.00
______________________________________
*minimum R, critical value for indefinite lateral extent.
TABLE 2
______________________________________
CRITICAL PARAMETERS, One Surface Free
R "a"
______________________________________
1,100.6.sub.5 *
2.682.sub.5
1,120.8 3.00
1,223.5 3.50
1,528.8 4.00
______________________________________
*minimum R, critical value for indefinite lateral extent.
The numerical factors which permit a direct connection between the "a"
parameter and the actual lateral dimensions depend on the geometrical form
of the repeating lateral cells. Three cell plans have received extensive
theoretical and practical treatment:
I. parallel Vertical Walls
II. Square Cell
Iii. hexagonal Cell (Equilateral Triangle is special case of Hexagonal
pattern) The results obtained may be summarized as follows, where "d" is
the depth of the circulating cell, and the values of "a" are to be
selected from the table above in accordance with a calculation of the
Rayleigh number and a determination of whether both surfaces are non-slip
or one is free.
I. parallel Vertical Walls:
L (fig. 4) (minimum lateral distance which will permit natural convection
cell to form under conditions specified) = 2 .pi. d/a
Ii. square Cell:
L (fig. 6) (side of minimum cell which will permit natural convection
pattern) = 2 .sqroot. 2 .pi. d/a
Iii. hexagonal Cell:
L (fig. 3) (side of minimum cell which permits circulation) = 4/3 .pi. d/a
(The side of the minimum equilateral triangle is .sqroot. 3 L calculated
for the hexagon, this makes the altitude come out the same as the critical
spacing for the parallel walls case above.) Rectangles other than squares
can be specified using the following notation:
Let L (FIG. 7) be the short side of the rectangle, and let r .times. L, r
.gtoreq. 1, be the long side. Then:
L (length of short side consistent with natural convection)
##EQU7##
This last result actually includes cases I and II above. When r > 1, we'll
recover the expression for parallel vertical walls, and when r = 1,
(square) we'll obtain the expression appropriate to the square. This is
obvious from the results published by Pellew & Southwell, and by
Chandrasekhar, and by ohters. See Pellew, Anne, and Southwell, R. V.,
Proc. Roy. Soc., Series A 176, 312-343, (1940). (There is a misprint
(factor of .pi. 2) in equation 61 in this paper).
For all of these systems, if real boundaries that approach the size of the
natural cell are added, the convective circulation will be inhibited. Or,
to look at it another way, the minimum Rayleigh number needed for
convection to occur will be driven up to artificially high values if we
introduce closely spaced lateral boundaries. In actual tests, Heitz and
Westwater found that the critical Rayleigh number associated with square
cells rose from approximately 2 .times. 10.sup.3 when the depth was half
the width, to approximately 3 .times. 10.sup.5 when the depth was four
times the width. See Heitz, W. L., and Westwater, J. W., Trans. of ASME
(J. of Heat Transfer), May 1971, 188-196.
There may be practical reasons for using hexagons or triangles as the plan
figure for the vertical dividers. Structural considerations, the efficient
use of materials and the like may be important. And further, less easily
described geometrical shapes designed to fill the plane might also be
chosen, perhaps because they are easy to handle or assemble. The important
consideration in terms of interfering with natural convection is that the
width of the figure should be the order of twice the depth .times. .pi./a,
where "a" is to be taken from the table above. The system of dividers,
whether cellular (FIGS. 3, 6 and 7), horizontal (FIG. 2), or vertical
spacers (FIG. 4) must have excellent light transmission characteristics.
Further, as has been noted, a vapor barrier across the top of the whole
pond is highly desirable.
FIG. 5 shows a further variation of the basic ideas already presented. In
this concept, the high viscosity fluid 58 is to be bagged in transparent
containers 60 which will then be stacked in a suitable crib or tank 55 to
form the solar pond 56. Two of the principal dimensions of the bags might
be very large as long as the other is sufficiently small to inhibit
circulation under the design conditions. It seems most likely that the
bags will be made wide and long and that to inhibit circulation, the
height of the bags "d" will have to be limited in accordance with the
critical Rayleigh number for a fluid layer of indefinite lateral extent.
That is, the height of each bag will be more or less the same as the
vertical spacing that would have been chosen for the horizontal sheets
shown in FIG. 2.
Bagged high viscosity fluids as envisaged here should not be confused with
the transparent water bags used in the patented SKYTHERM system developed
by Harold Hay of Phoenix, Ariz. In this SKYTHERM system, and in other
concepts that are currently under study, the water or other fluid is used
for its thermal mass and/or possibly its infra-red opacity (depending on
the design and purpose of the system). However, it is allowed to convect
freely; and in some concepts it is even pumped from storage to the shallow
pond and back again, a procedure that would require prohibitive amounts of
energy if the viscosity were the order of 100,000 centipoises or more!
A great variety of thickening agents and techniques are available for
thickening the fluid that is to be used in the viscosity stabilized or
gelled solar pond. High polymers dissolved in the fluid can produce
substantial viscosity increases at relatively modest concentrations.
Alternatively, two inherently fluid but immiscible substances, oil and
water, can be made to exhibit the characteristics of a gel if combined and
emulsified in appropriate proportions along with suitable low or moderate
molecular weight emulsifying agents. In addition, fluid bodies can be
gelled using certain medium molecular weight polymers which have
substantial segments that are respectively oil soluble and water soluble.
The principal requirements for the purpose at hand are that the gelled
bodies should be exceptionally clear, capable of transmitting most of the
light that falls on them, that they retain their thickened consistency at
temperatures of 40.degree. C. and preferably above, and finally, in the
interest of conserving the heat absorbed by the blackened receiver, they
should have a high infra-red absorbance and a poor thermal conductivity.
Descriptive examples of three kinds of gels which are more than 50%
aqueous follow below.
The formulation and preparation of transparent mineral oil/water gels has
been described by W. R. Markland, D. F. Doca and P. Tusa, U.S. Pat. No.
3,228,842; Jan. 11, 1966; assigned to Chesebrough-Pond's Inc. A
transparent gel that solidifies just below 60.degree. C. has the following
formulation: mineral oil (visc. 90 Saybolt seconds) 20%; ethoxylated
hydrogenated lanolin (20 mol ethylene oxide adduct) 15%; ethoxylated
oleylcetyl (7:3) alcohols (4 mol ethylene oxide adduct) 10%; preservative
and/or other minor ingredients as needed--typically less than 2%; water to
make 100%--53 to 55% depending on usage of minor ingredients. The gel is
prepared by mixing the oil, the emulsifiers and the preservative and
heating these with stirring to about 77.degree. C. Water, preheated to
this temperature, is added slowly with stirring and the resulting mixture
is cooled with stirring to about 60.degree. C. Water lost during cooling
is added at this point and after stirring, the mixture is poured out and
allowed to solidify.
The variety of polymeric agents that may be used to gel aqueous bodies is
truly astonishing. Natural materials such as gum arabic, locust bean gum,
algin, starch, and gelatin are potentially useful for the present purpose
if sufficiently refined and clarified. Further there is a large class of
semi-synthetic materials, the principal articles of commerce being water
soluble derivatives of cellulose and of starch. And finally, there are
purely synthetic materials such as polyvinyl pyrrolidone, polyvinyl
alcohol, carboxy vinyl polymer, polyacrylic acid, polyacrylamide, ethelene
oxide polymers, and no doubt others. The list above is not meant to be
limiting but merely illustrative. A more extensive but still br | | |
