Abstrict A solid methyl cellulose desiccant wherein from about 0.6 to 1.8
preferably 0.9 to 1.5 of the available hydroxyl groups of the anhydroglucose
units of the cellulose entity have been replaced by methyl, or methyl
and hydroxy alkyl or carboxy alkyl groups, if any, containing from
2 to about 4 carbon atoms, e.g. hydroxy ethyl, hydroxy propyl, carboxyl
methyl and the like. In the solid methyl cellulose desiccant, at
least one of the substituting groups must be methyl, with the remainder
of the substituting groups, if any, hydroxy alkyl or carboxy alkyl,
or both; preferably carboxy methyl, if any. At least one-half of
the substituting groups are methyl, and preferably at least two-thirds
of the substituting groups are methyl; the balance of the substituting
groups, if any, being hydroxy alkyl or carboxy alkyl groups. Methyl
cellulose of this character can be contacted with a water-containing
stream at a temperature below which it will have great affinity
for and will absorb significant amounts of water; and conversely
above this temperature the methyl cellulose will not only lose this
affinity but in addition may give up a large portion of any water
it had previously absorbed.
Claims Having described the invention, what is claimed is:
1. A process for the dehydration of a fluid stream with a desiccant,
and regeneration of said desiccant which comprises contacting said
fluid stream with a desiccant at a temperature below its syneresis
temperature, the syneresis temperature of said desiccant ranging
from about 100.degree. F. to about 195.degree. F., said desiccant
being characterized as solid methyl cellulose wherein from about
0.6 to about 1.8 of the available hydroxyl groups of the anhydroglucose
units constituting the basic cellulose entity have been replaced
by methyl and other hydroxy alkyl or carboxy alkyl groups containing
from 2 to about 4 carbon atoms where at least one-half of the substituting
groups are methyl, and heating said desiccant above the syneresis
temperature, said solid methyl cellulose desiccant having the capacity
to lose its affinity for water, and to release water when heated
above its syneresis temperature.
2. The process of claim 1 wherein the desiccant is solid methyl
cellulose wherein at least two-thirds of the substituting groups
are methyl, where all of the substituting groups are not methyl.
3. The process of claim 1 wherein the desiccant is solid methyl
cellulose which contains carboxyl methyl groups.
4. The process of claim 1 wherein the desiccant is solid methyl
cellulose and essentially all of the substituting groups are methyl.
5. The process of claim 1 wherein the syneresis temperature of
the solid methyl cellulose desiccant ranges from about 140.degree.
F. to about 160.degree. F.
6. A process for drying air with a desiccant, and regeneration
of said desiccant which comprises contacting said fluid stream with
a desiccant at a temperature below its syneresis temperature, the
syneresis temperature of said desiccant ranging from about 100.degree.
F. to about 195.degree. F., said desiccant being a solid methyl
cellulose wherein from about 0.6 to about 1.8 of the available hydroxyl
groups of the anhydroglucose unit constituting the basic cellulose
entity have been replaced by methyl, or methyl and other hydroxy
alkyl or carboxy alkyl groups containing from 2 to about 4 carbon
atoms where at least one-half of the substituting groups are methyl,
and heating said desiccant above the syneresis temperature, said
solid methyl cellulose desiccant having the capacity to lose its
affinity for water, and to release water when heated above its syneresis
temperature.
7. The process of claim 6 wherein the desiccant is solid methyl
cellulose which contains at least two-thirds of the substituting
groups as methyl, where all of the substituting group are not methyl.
8. The process of claim 6 wherein the desiccant is solid methyl
cellulose which contains carboxy methyl groups.
9. The process of claim 6 wherein the desiccant is solid methyl
cellulose and essentially all of the substituting groups are methyl.
10. The process of claim 6 wherein the syneresis temperature of
the solid methyl cellulose desiccant ranges from about 140.degree.
F. to about 160.degree. F.
Description Many industrial processes produce water as a by product of a chemical
reaction, the water as a contaminating constituent of a fluid stream
from which it must be separated, and removed. In some natural environments
too water is a desirable product, and its recovery is a worthwhile
objective in itself. For example, in the production of absolute
alcohol, e.g. ethyl alcohol from a constant boiling mixture of ethyl
alcohol and water, it is essential to remove the water from the
admixture to form this useful industrial chemical compound. Illustrative
also, in the production of methyl and ethyl cellulose water is produced
as one of several by-products, and water must be separated from
the reaction mixture to avoid further hydrolysis reactions which
produce additional undesirable by-products. Desiccants, as is known,
can be used for the separation of water from such streams, even
the separation and recovery of water from air, but desiccants are
not nearly so energy efficient as desired, particularly in todays
short energy supply situation.
Known desiccants include bauxite, inorganic oxide gels, notably
silica and alumina, silica-alumina, activated carbon, molecular
sieves and the like. Desiccants for the dehydration of liquids,
e.g. alcohols, are also well known and include such materials as
sodium sulfate, copper sulfate, zinc chloride, mercuric chloride,
potassium carbonate, carboxy methyl cellulose and the like. Solids
desiccants are generally used in palpable particulate form and include
such shapes as tablets, pellets, spheres and the like. Liquids,
e.g. ethylene glycol, have also been used as desiccants, or absorbents,
as for the dehydration of alcohols. In the use of desiccants for
most industrial purposes the dehumidification step is conducted
in a packed column and, when the desiccant is sufficiently wet that
its water adsorption capacity is impaired, it is revivified or regenerated
by contact with a dry gas, generally at elevated temperature. Generally
too, the dehydration and revivification steps are conducted in parallel
vessels to provide a continuous operation. The notoriously energy
inefficient portion of the operation resides in the revivification,
or regeneration of the desiccants. Large quantities of heat are
required to dry the wet desiccants, and one-step operations wherein
the wet desiccant is discarded are generally even more prohibitive.
It is, accordingly, the primary objective of the present invention
to provide a new and novel composition, or desiccant useful for
the dehumidification, or dehydration of wet materials contacted
therewith.
A specific objective is to provide a new and novel desiccant, particularly
useful for the dehumidification, or dehydration of fluids; notably
gaseous, liquid or mixed phase streams.
A further object is to provide a desiccant, or drying agent as
characterized which can be easily regenerated by low temperature
heat.
Yet another, and more specific object is to provide a desiccant,
and process utilizing said desiccant, which is suitable for the
extraction of water from a fluid, notably cool air, and the wet
desiccant thereafter heated to a higher, slightly elevated temperature,
notably by the rays of the sun, at which temperature the desiccant
will release its absorbed water to its surroundings and become dry,
as well as lose its normal afinity for water at the elevated temperature;
and which desiccant will regain its normal affinity for water on
being brought back to said lower temperature.
These objects and others are achieved in accordance with the present
invention which embodies a solid methyl cellulose desiccant wherein
from about 0.6 to about 1.8 preferably from about 0.9 to about
1.5 of the available hydroxyl groups of the anhydroglucose units
of the cellulose entity have been replaced by methyl, or methyl
and other hydroxy alkyl or carboxy alkyl groups, if any, containing
from 2 to about 4 carbon atoms, e.g. hydroxy ethyl, hydroxy propyl,
methyl hydroxy ethyl, methyl hydroxyl propyl, carboxy methyl, carboxy
ethyl, carboxy propyl groups, or the like. In the solid methyl cellulose
desiccant, at least one of the substituting alkyl groups must be
methyl, with the remainder of the substituting groups, if any, either
hydroxy alkyl or carboxy alkyl or both; preferably carboxy methyl,
if any. At least one-half of the substituting groups are methyl,
and preferably at least two-thirds of the substituting groups are
methyl; the balance of the substituting groups, if any, being hydroxy
alkyl or carboxy alkyl groups. Methyl cellulose of this character
can be contacted with a water-containing stream at a temperature
below which it will have great affinity for and will absorb significant
amounts of water; and conversely above this temperature the methyl
cellulose will lose this affinity and not absorb any significant
amount of water. This temperature, defined as the syneresis temperature
of the methyl cellulose, can be varied over a range of from about
100.degree. F. to about 195.degree. F., more often over a range
of from about 140.degree. F. to about 160.degree. F., dependent
upon the degree of substitution and, to some extent, upon the nature
and proportion of the substituent groups themselves.
Methyl cellulose, as known, is a cellulose ether comprised of linear
chains of .beta.-anhydroglucose rings, manufactured from cellulose.
Cellulose, in its natural state is polysaccharide composed of a
variable number of individual anhydroglucose units linked together
through the 1 and 4 carbon atoms with a .beta.-glucosidic linkage
characterized, e.g., by Haworth et al, as follows: ##STR1## Haworth,
W. N., Hirst, E. L., and Thomas, H. A., "Polysaccharides, Part
VII," J. Chem. Soc. 824(1931). The hydroxyl groups of cellulose
are the primary reaction sites and, it will be noted, comprise,
with the moieties to which they are associated, a primary alcohol
attached to the number 6 carbon atom and secondary alcohols attached
to the number 2 and 3 carbon atoms, these being sites which can
react to form ethers of cellulose. The hydroxyl groups of a monomer
unit (N-2), through the exertion of hydrogen bonding, cause considerable
intermolecular attraction of the Van der Wall type between chains,
thus lessening the reactivity normally expected of alcohols until
such time as the cellulose has become solvated.
The number of monomer units in a single chain of cellulose can
range from a few hundred, or less, to several thousand, e.g., 30
to 5000 or more, and the structure of cellulose is basically crystalline
in nature, partially due to the stiff glucosidic chains, the presence
of the hydroxyl groups, and because of the length and flexibility
of the chains which results in their entanglement. Some amorphous
regions, however, do exist in the cellulose chain. Due to the intermolecular
forces holding the molecules together, solvent penetration is more
difficult than in lower molecular weight compounds but various techniques
are well known to the art for hydrolysis or solvation of cellulose,
and after solvation the primary and secondary hydroxyl groups become
accessible so that ethers can be formed. Chemically, after solvation
of the cellulose, cellulose reacts basically in the same manner
as primary and secondary alcohols. The average number of hydroxyl
groups replaced, based on the three available hydroxyl groups per
anhydrogluclose unit, determines the degree of substitution (D.S.)
on the chain. A fully substituted cellulose derivative, e.g., would
have a degree of substitution of 3.0 whereas a degree of substitution
of 0.6 would mean that an average of twenty percent of the available
hydroxyl reaction sites have been replaced while eighty percent
remain as free hydroxyl groups; or, a degree of substitution of
1.8 would mean that an average of sixty percent of the available
hydroxyl reaction sites have been replaced while forty percent remain
as free hydroxyl groups. In accordance with the present invention,
e.g., a solid methyl cellulose desiccant having a D.S. of 0.6 would
have an average of twenty percent of the available hydroxyl reaction
sites substituted by methyl, or at least one-half (10%) and preferably
two-thirds (16%) of the available hydroxyl reaction sites substituted
by methyl, with the balance of the substituted reaction sites containing
hydroxyl alkyl or carboxy alkyl groups. A solid methyl cellulose
having a D.S. of 1.8 on the other hand, would have an average of
at least 60 percent of its available hydroxyl reaction sites substituted
by methyl, or at least one-half (30%), and preferably at least two-thirds
(40%) of the available hydroxyl reaction sites substituted by methyl,
with the balance of the substituted hydroxyl reaction sites containing
hydroxyl alkyl or carboxy alkyl groups. Substitution occurs most
readily within the amorphous regions of the cellulose, and the degree
of substitution has marked effect on the solubility of the of the
substituted cellulose; the more substituted the chain, the more
soluble the substituted product or derivative up to a certain limiting
number above which the nature of the solubility changes such that
very highly substituted products may not be soluble in water but
may be soluble in organic solvents.
Strong bases can thus be used to partially solvate the natural
cellulose, causing it to swell, thus allowing for easier penetration
by reactants for formation of ethers. The swelling or etherifying
agents disrupt hydrogen bonding and other secondary forces bonding
the hydroxyl groups and thereby increases the uniformity of access
so that the reactions can be produced. Some common swelling agents
are alkali metal hydroxides, e.g., the hydroxides of potassium,
lithium, cesium, rubidium, and the like, liquid ammonia, trimethylsulfonium
hydroxide, guanidinium hydroxide, cupraammonium hydroxide, trimethylbenzylammonium
hydroxide, and the like. Alkali or alkaline earth metal hydroxides
are particularly preferred and are commonly used to effect partial
solvation because it is low in cost and provides desired uniform
product distribution with minimum degration.
The presence of the methyl groups in the solid methyl cellulose
desiccant of interest is essential in order to develop a low gelation
temperature, this being essential to develop in the solid methyl
cellulose desiccant of interest the high affinity for water at low
temperature. Within the cellulose chain the methyl group is unique
in that is it ideally sized to provide hydrogen-bonding adsorption
sites, or "voids" sufficient for the adsorption, and retention
of water below a given syneresis temperature. The presence of some
hydroxyl alkyl or carboxy alkyl groups is permissible, and in some
instances desirable, but because of their size, the distance between
the cellulose chains is increased and so, consequently, is the ability
of the methyl cellulose to reject the water hydrating the hydroxyl
groups to obtain gelation. Moreover, not only does increased substitution
per se increase gelation temperature, but the increased size of
the substituting groups themselves produces increased gelation temperature
which is undesirable.
The solid methyl cellulose desiccant of this invention is thus
a unique specie within the class of methyl cellulose ethers, and
it has unique properties as contrasted with known members of this
class which are prepared for use in solution, and have commercial
value because of this characteristic. The following energy efficient
usages thus exemplify the solid methyl cellulose desiccants of this
invention, to wit:
(1) A fermentation broth can be distilled to recover a constant
boiling mixture, or azeotrope, containing 95.57 percent ethyl alcohol
in water. This mixture contacted with a solid methyl cellulose desiccant
(DS=1.8), the substituting groups being all methyl, to preferentially
absorb the water away from the mixture. The wetted solid desiccant
can then be separated from the essentially pure, absolute alcohol,
dried in a stream of air, or in the ambient atmosphere in sunlight,
and then reused.
(2) A wet product vapor stream from a vessel within which conventional
methyl cellulose is being prepared can be contacted with the desiccant
of (1), supra, and the water preferentially absorbed. Water thus
removed from the product stream results in the suppression of hydrolysis
reactions which normally lead to the production of undesirable by-products,
and consequently, to the consumption of costly reactants.
(3) Water can be obtained from cool night air by passage over the
desiccant of (1), supra, packed within a tube, to saturate it with
water. By exposure of the tube of wetted desiccant to the morning
sun, the temperature of the desiccant exceeds its syneresis temperature,
about 140.degree. F., and the water is released by the desiccant.
An air stream passed through the tube can pick up the water, and
the air then cooled in a water condenser to recover the water; thus
making water available in the desert using the sun as the only energy
source.
The polymeric, solid methyl cellulose desiccant exemplified in
(1), (2) and (3), supra, will thus adsorb water very rapidly at
temperatures below 120.degree. F. In contrast, when the temperature
of the desiccant is increased to 140.degree. F., the desiccant suddenly
and completely loses its ability to adsorb and retain water, but
rather releases water to its surroundings. It is believed that this
absorption-desorption phenomenon results from the ability of the
polymer molecules to hydrogen bond to each other above the syneresis
temperature, while below the syneresis temperature the polymer preferentially
bonds to water molecules. The importance of this phenomenon is that
the polymeric, solid methyl cellulose of this invention can be dried
at very low temperatures, particularly at solar temperatures such
as those available during normal daylight hours. Even though other
desiccants are known therefore, high temperatures are required for
drying such desiccants this making conventional heating, or refrigeration
which may use such desiccants, at best, only solar assisted. The
present unique methyl cellulose desiccant makes possible true solar
heating, and refrigeration.
The process of this invention, at the heart of which lies the novel
solid methyl cellulose desiccant, requires generally an initial
contact of a cool moisture-containing stream of air with said desiccant
at below its syneresis temperature at which point in time the desiccant
will adsorb and retain water, with concurrent adiabatic heating
of the dehumidified air. The warm dehumidified air, as would be
expected, can be used to maintain an enclosure, or facility, e.g.
residence, plant work space or office, at comfortable temperature
during cold months, e.g. as in winter. In a subsequent step, where
refrigeration is desired, as in summer, the desiccant can be used
to dry ambient air with the accompanying generation of heat. The
hot dry air is then cooled by exchange with ambient air, and the
cool dry air then passed through a water evaporator which further
cools down the air to a comfortable temperature for use in cooling
an enclosure, or facility, e.g., residence, plant work space or
office. The desiccant can then be dried by raising its temperature
in a solar collector to above the gel temperature.
The FIGURE discloses a house incombination with a ventilation system
including the methyl cellulose desiccant.
The process, and the principle of its operation, will be better
understood by reference to the following more detailed description
of a preferred emboiment, and to the attached FIGURE to which reference
is made as the description unfolds. This embodiment shows methods
by means of which a typical residence can be heated or refrigerated.
Referring to the FIGURE there is depicted a typical residence 10
or house with roof 11 within which is mounted one or a plurality
of tubes 12 packed with the solid methyl cellulose desiccant of
this invention, one segment of tubes of which are shown for simplicity
of illustration. A pump 13 located at a corner of the house can
withdraw ambient, cool air from the atmosphere and pump same via
line 14 into a header 15 from which it is introduced through the
series of tubes 12 packed with the solid methyl cellulose desiccant
of this invention. The air, on passage through the desiccant filled
tubes 12 has its water removed, or is dried, while simultaneously
the air is warmed. The warm dry air is discharged from tubes 12
into the header 16 and then into the house 10 via line 17 and manifold
18. The warm air is distributed throughout the house by ductwork
(not shown). The desiccant can be regenerated by reversal of the
air flow by withdrawal of air from ambient and feeding same into
line 17 at a period when the house need not be warmed, or while
another unit (not shown) is supplying warm air to the interior of
the house.
By precooling the warm dry air, as by heat exchange means not shown,
e.g. with ambient air, with subsequent passage of the cool dry air
into the house 10 through a humidifier 18 on the other hand, the
cool dry air can be further cooled to refrigerate the house, as
in summer when cooling is needed.
Integration of the tubes 12 into conventional solar collection
panels (now shown) to permit the rays of the sun to play upon the
tubes during the period of regeneration provides an energy efficient
means to regenerate the desiccant. It is thus but a simple matter
to shade the tubes during the adsorption portion of the cycle, and
to expose the tubes to the sun during the regeneration portion of
the cycle to desorb the moisture from the desiccant.
Whereas it is known to use a drying agent, or desiccant to initially
dry humid air, which causes adiabatic heating, with subsequent cooling
of the adiabatically heated air by heat exchange with ambient air,
and final vaporization of water into the air with some type of humidifier
to obtain refrigeration, the difficulty with such processes is that
once the desiccant or drying agent is saturated with water it has
to be dried with high temperature heat in order to make the desiccant
give up its moisture. Most systems must therefore burn natural gas,
butane or stream to regenerate the drying beds. Quite obviously
if high temperature heat must be used to remove the water from the
desiccant, this same heat could more advantageously by used to run
an ammonia evaporation refrigeration system or conventional cooling,
and consequently, present desiccants do not provide suitable means
to make solar heating, and refrigeration a reality. Real world economics,
today requires a lessened use of, or avoidance of fossil fuels as
energy sources for low cost heating, and regrigeration.
In the process of this invention, on passage of air over the solid
methyl cellulose derivative (D=1.8), the desiccant, for example,
picked up 17% moisture when exposed to 80% relative humidity (RH)
for 24 hours at room temperature, but lost moisture down to 3% while
still exposed to 80% RH but with the temperature increased to 160.degree.
F. A second derivative exposed at the same conditions picked up
14% water at ambient temperature but when heated to 160.degree.
F. while still at 80% RH lost water down to 1%. On both samples
the cycle was repeated several times with almost identical gain
and loss of moisture due obviously only to changes in temperature.
This invention makes solar refrigeration truly solar. No high temperature
gas drier is needed to regenerate the bed because the desiccant
of this invention will obviously dry at temperatures easily available
in the day time in summers to the peak of the roof which may reach
temperatures as high as 160.degree. F. to 180.degree. F., depending
on the temperatures of the air entering the tubes at the bottom
and on its relative humidity. The lower the initial relative humidity
the lower will be the rise in temperature of air reaching the plenum
at the peak of the roof.
The hot-dry air coming to the peak of the roof from the solar collectors
on the roof not facing the sun (or at night) will pass into a heat
exchanger (not depicted in the drawing) whose purpose will be to
exchange heat with ambient air to cool down the hot air as much
as possible. The hot-dry air should cool down to within 5.degree.
to 10.degree. F. of ambient air. This may in some cases require
baffles on the inside of the heat exchanger and radiation fins on
the outside, and such apparatus is not very difficult to fabricate.
Once the dry air is cooled to within a few degrees above ambient
it is sucked into a blower which forces it to go thru moistened
pads to humidify it and to cause cooling of the air. Again the temperature
that the humidified air will drop to will depend on how much water
remains in the air and its temperature before entering the humidification
chamber. Usually the air temperature will drop to 60.degree. to
65.degree. F. This cool air will generally be forced either into
the house if cooling is required or into some kind of energy storage
chamber such as stones or gravel. This energy storage material may
be cooled down at night and thru the day the desiccant could be
dried by the sun. Cold air may be pulled from the storage chamber
whenever refrigeration is required. On the other hand, drying of
the air can be done during part of the day when the roof is in the
shade and on the other part of the roof the desiccant could be dried
simultaneously by the sun, which would require less desiccant.
It is apparent that certain variations can be made without departing
the spirit and scope of the present invention. In its essence, the
present invention is based on the discovery of unique solid methyl
cellulose desiccants which can absorb large quantities of water
at temperatures below a syneresis temperature ranging from about
100.degree. F. to about 195.degree. F., preferably about 140.degree.
F. to about 160.degree. F., depending upon its composition. Below
this temperature it will hold the water tightly, and above this
temperature it will reject the water so that it can be easily dried
by application of low temperature heat. In particular, the desiccant
can be used for drying air below the syneresis temperature; and
by virtue of its nature the desiccant will undergo a solid state
transition when the temperature exceeds the syneresis temperature
which causes it to lose its affinity for water almost completely
such that on the application of low temperature heat, as when the
sun is shining and the temperature reaches syneresis temperature,
the water is quickly swept away by an air stream, drained off or
decanted from the desiccant. This unique desiccant can provide large
savings in both energy and capital costs, and is particularly useful
for solar heating, and refrigeration applications. |