Abstrict This invention relates to the use of desiccants in conjunction
with an open oop drying cycle and a closed loop drying cycle to
reclaim the energy expended in vaporizing moisture in harvested
crops. In the closed loop cycle, the drying air is brought into
contact with a desiccant after it exits the crop drying bin. Water
vapor in the moist air is absorbed by the desiccant, thus reducing
the relative humidity of the air. The air is then heated by the
used desiccant and returned to the crop bin. During the open loop
drying cycle the used desiccant is heated (either fossil or solar
energy heat sources may be used) and regenerated at high temperature,
driving water vapor from the desiccant. This water vapor is condensed
and used to preheat the dilute (wet) desiccant before heat is added
from the external source (fossil or solar). The latent heat of vaporization
of the moisture removed from the desiccant is reclaimed in this
manner. The sensible heat of the regenerated desiccant is utilized
in the open loop drying cycle. Also, closed cycle operation implies
that no net energy is expended in heating drying air.
Claims What is claimed is:
1. A method of drying a crop comprising the steps of continuously
passing a regenerated, liquid desiccant from a storage tank through
an absorption column and then through a first heat exchanger back
to said tank; passing hot, dry air through a crop bin containing
said crop for absorbing moisture therefrom; passing the moist exit
air from said bin to said absorption column containing said regenerated,
liquid desiccant for removing the moisture from said moist air;
passing the exit air from said absorption column through said first
heat exchanger for the heating and drying of said air before it
is again passed through said bin in a closed-loop fashion; removing
said desiccant from said storage tank after it becomes used and
saturated with moisture and regenerating it comprising the steps
of passing it through a condenser for preheating thereof, through
a second heat exchanger for further preheating therof, and through
heating means for regenerating (further heating) said saturated
desiccant; then passing said regenerated desiccant exiting from
said heating means through means for separating the water vapor
therefrom; passing said separated water vapor through said condenser
where it is condensed to water and passed to a drain, said water
vapor being condensed in said condenser serving as an energy recovery
mechanism to provide for said preheating of said saturated desiccant
passing therethrough to said heating means, passing the now dry,
hot, regenerated desiccant from said moisture separating means through
said second heat exchanger for said further preheating of said saturated
desiccant prior to its passing through said heating means and passing
the regenerated, dry desiccant exiting from said second heat exchanger
to said desiccant storage tank, and repeating all of said steps
as many times as necessary to dry said crop to a desired dryness.
2. The method set forth in claim 1 wherein said heating means
is a solar collector and an electrical heater connected in series
between said second heat exchanger and said moisture separating
means, said regenerated, hot, dry desiccant exiting from said second
heat exchanger is passed through said first heat exchanger before
being passed to said storage tank, and further including the steps
of passing ambient air through said first heat exchanger for the
heating and drying thereof and then through said crop bin to the
atmosphere in an open loop fashion while at the same time the passing
of said desiccant through said absorption column is stopped during
the time said saturated desiccant is being regenerated and said
crop bin is supplied drying air in said open loop fashion.
3. The method set forth in claim 2 wherein said liquid desiccant
is a lithium chloride solution and said moisture separating means
is a stripping column.
4. The method set forth in claim 2 wherein said liquid desiccant
is a lithium chloride solution and said moisture separating means
is an evaporator.
5. The method set forth in claim 1 wherein said crop bin is an
open-ended, two-stage column dryer of two stacked bins through which
grain to be dried is continuously passed, said regenerated hot,
dry desiccant exiting from said second heat exchanger is passed
through a third heat exchanger before being passed to said storage
tank, and further including the steps of passing ambient air through
said third heat exchanger for the heating and drying thereof and
then passing it through a first bin of said two-stage column dryer
in an open-loop fashion for preheating the grain passing through
said first bin, further drying said grain exiting from said first
bin in a second bin of said two-stage column dryer as said grain
passes therethrough by the hot dry air from said first heat exchanger
which air is passed through said second bin in said closed-loop
fashion, wherein said bins are supplied with drying air in a continuous
manner and said used desiccant is regenerated in a continuous manner.
6. The method set forth in claim 5 wherein said moisture separating
means is a stripping column, said heating means is a solar collector
and an electrical heater connected in series between said second
heat exchanger and said stripping column.
7. The method set forth in claim 6 wherein said liquid desiccant
is a lithium chloride solution.
Description The United States produces large volumes of crops for food and
livestock feed every year. These crops require drying before temporary
or long term storage. The degree of crop drying depends upon the
condition of the crop at harvest and the intended end use for the
crop. Energy required to dry crops using conventional crop drying
equipment is typically about 2200 Btu per pound of water removed
from the crop. The 2200 Btu includes energy required for blowers
to circulate air as well as the thermal energy used to heat the
drying air and the crop. Considering just the grain crops, an estimate
of the annual energy requirement for drying crops may be made assuming
that roughly 10% of weight of the harvested grain crops is excess
moisture which must be removed and using 60 lb. as a weight equivalent
for one bushel of grain. An energy requirement of 1.2.times.10.sup.14
But would be expended annually if the total harvest of grain crops
were submitted to drying.
Conventional crop drying equipment dries crops in an open loop
cycle wherein ambient air is heated to decrease the relative humidity
of the air and then drives the heated, low humidity air through
the crop where moisture is absorbed from the crop. The air is then
expended to the atmosphere and carries with it the energy used to
vaporize moisture in the crop and that used to increase the air
temperature.
Thus, the energy requirements for crop drying is substantial and,
in view of potential shortages in fossil fuel energy sources, concepts
for conserving energy expenditures for crop drying are desirable.
The present invention was conceived to meet this need in a manner
to be described hereinbelow.
SUMMARY OF THE INVENTION
It is the object of the present invention to provide a method and
apparatus for the drying of harvested crops in such a manner that
a substantial saving in the required energy therefor is effected.
The above object has been accomplished in the present invention
by utilizing desiccants in conjunction with an open loop drying
cycle and a closed loop drying cycle to reclaim the energy expended
in vaporizing moisture in harvested crops. In the closed loop cycle,
the drying air is brought into contact with a desiccant after it
exits a crop drying bin. Water vapor in the moist air is absorbed
by the desiccant, thus reducing the relative humidity of the air.
The air is then heated by the used desiccant and returned to the
crop bin. During the open loop drying cycle the used desiccant is
heated and regenerated at high temperature, driving water vapor
from the desiccant. This water vapor is condensed and used to preheat
the dilute (wet) desiccant before heat is added from the external
source. The latent heat of vaporization of the moisture removed
from the desiccant is reclaimed in this manner, and the sensible
energy contained in the regenerated hot desiccant is used in the
open loop drying cycle after which it is then recycled through the
closed loop as many times as desired until the crop in the drying
bin has been dried to a desired state .
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1a is a graph illustrating the operating range of calcium
chloride when it is utilized as a desiccant;
FIG. 1b is a graph illustrating the operating range of lithium
chloride when it is utilized as a desiccant;
FIG. 2a is a graph illustrating the closed loop drying cycle of
one system of the present invention;
FIG. 2b is a graph illustrating the corresponding changes in the
desiccant concentration and temperature with respect to FIG. 2a;
FIG. 3a is a graph illustrating the open cycle drying operation
of another system of the present invention;
FIG. 3b is a graph illustrating the regeneration operation of the
desiccant in the system referred to in FIG. 3a;
FIG. 4 is a schematic diagram of one embodiment of the present
invention utilizing a liquid desiccant to which FIGS. 2a and 2b
relate;
FIG. 5 is a schematic diagram of another embodiment of the present
invention which is a modification of the system of FIG. 4 to which
FIGS. 3a and 3b relate; and
FIG. 6 is a schematic diagram of still another embodiment of the
present invention utilizing an evaporator as a desiccant regenerator.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Desiccants are hygroscopic chemical substances that have large
affinity for water. Although desiccants remove water by a variety
of mechanisms, the action of many desiccants may be understood in
terms of water vapor partial pressure. Due to the difference in
the partial pressure of water vapor in the desiccant and the material
to be dried water will deffuse from the material being dried to
the desiccant until a dynamic equilibrium is reached. This occurs
when the two substances attain the same partial pressure of water.
At this point no net transfer of water takes place. The performance
of the desiccant may be evaluated in terms of efficiency and capacity
that are defined as follows. Drying efficiency is the fraction of
total water input that the desiccant removes. Drying capacity is
the quantity of water that a unit mass of desiccant can take up
before losing drying efficiency.
Examples of liquid desiccants that can be utilized in the present
invention include deliquescent salt solutions such as calcium chloride
or lithium chloride in water as well as organic compounds such as
glycol, glycerine and sulfuric acid. These are all liquid at all
ordinary ranges of temperature and dilution. When in solution, deliquescent
compounds will obviously have lower drying efficiency and capacity
than the same anhydrous salt, but the much greater ease of handling
the liquid solution makes the solutions preferable where very low
humidities are not required as is the case in crop drying. The anhydrous
liquids (glycol for example) can produce nearly complete dehydration,
but large quantities of the drying agent must be used due to low
drying capacity, and complete regeneration is usually difficult.
One of the greatest advantages of the liquids is that a desired
relative humidity of the drying air can be maintained with very
close control regardless of inlet moisture conditions. This is accomplished
by simply maintaining the dehydration solution at the proper concentration
and temperature or by varying the flow rate of the desiccant.
Since glycerine is sensitive to thermal decomposition and must
be regenerated in vacuum, and sulfuric acid and glycol are toxic,
the preferred liquid desiccants are lithium chloride and calcium
chloride solutions. However, it can be seen from FIG. 1a of the
drawings that the operating range of calcium chloride, illustrated
by the shaded area thereof, is substantially limited compared to
that of lithium chloride as illustrated by the shaded area of FIG.
1b, such that the lithium chloride is the preferred desiccant of
these two salts.
FIGS. 4 5 and 6 of the drawings illustrate three respective embodiments
of the present invention in which a liquid desiccant is utilized,
and the details of these various embodiments will now be described.
A schematic diagram of one system of the present invention using
liquid desiccants (lithium chloride solution, for example) to dry
a crop contained in a drying bin is illustrated in FIG. 4 of the
drawings. It should be understood that open cycle drying is effected
through the crop bin at the same time as the dilute desiccant is
being regenerated in a regeneration loop of the system after which
closed cycle drying is effected through the crop bin to complete
the total drying cycle of the system.
Regenerated, hot desiccant from a storage tank 16 is continuously
fed by means of a pump 17 and a valve 20 to an absorption column
5 positioned in a closed-loop air line with a blower 3 heat exchanger
19 and a crop bin 1. The moist air being moved by the blower 3
from the bin 1 comes into contact with the hot desiccant in the
column 5 and is absorbed thereby. The dry air from the column 5
then flows back to the bin 1 through the heat exchanger 19 where
it is heated to a temperature of 90.degree. F. and dried to a relative
humidity of 40%, for example, prior to its flow through the bin.
During this closed cycle operation the valves 8 and 18 are closed
since the regeneration loop is not being used at this time.
The desiccant from the column 5 is then pumped by means of a pump
6 through a valve 7 and the heat exchanger 19 back to the storage
tank 16. In order to regenerate the desiccant after it becomes saturated
(diluted), it is pumped from the storage tank 16 by means of the
pump 17 through a valve 18 to a condenser 9 thence through a heat
exchanger 10 to a solar heater 12 and then through an electrical
heater 13 to a stripping column 14 where the water vapor is drawn
off the now hot, dilute desiccant and is then fed through the condenser
9. During regeneration of the dilute desiccant, the valves 7 and
20 are closed and the bin 1 is then coupled in an open-cycle drying
mode in a manner to be described hereinbelow. The hto water from
the condenser 9 is then fed to a drain. It should be noted that
the hot water vapor fed to the condenser 9 preheats the dilute cool
desiccant flowing therethrough, and the desiccant is also preheated
in the heat exchanger 10 (before passing through the solar unit
12) by the hot regenerated desiccant pumped from the column 14 by
a pump 15 to the unit 10 after which the hot desiccant is returned
to the storage tank 16 by way of the valve 8 and the heat exchanger
19. A blower 22 is provided which circulates air flow between the
regeneration column 14 and the condenser 9.
It should be noted that should the solar unit 12 not be required
or desired in the operation of the system then it can be bypassed
by means of a valve 11 and the dilute desiccant passed directly
to the heater 13.
The crop bin 1 is constructed of plexiglass material, for example,
and it is approximately 12 in. in diameter by 42 in. high. Air flow
is upward, with the crop to be dried resting on a perforated metal
support plate. The bin is removable from the system.
The air ducting is 3 in. i.d. PVC pipe, for example. Nominal air
velocity is 6.7 ft/sec. The blower 3 is a squirrel cage unit with
a 24 Vdc motor and permits variable speed operation with air for
up to 40 cfm for closed cycle drying in which the air valves 2 and
21 are closed and air valve 4 is opened. For open cycle drying,
the air valves 2 and 21 are opened and the air valve 4 is closed,
and air is drawn in upstream of the blower 3 passes through the
heat exchanger 19 and the crop bin 1 and is discharged from the
top plenum of the crop bin to the atmosphere.
The absorption column 5 consists of a 24 in. height of 1 in. o.d.
Raschig rings cut from PVC pipe. The packing is arranged randomly
(dumped) and is supported on a perforated plastic plate giving a
47% flow area, for example.
The inlet desiccant flow to the column 5 is at a rate of 45 pounds
per hour at a temperature of 90.degree. F., for example, and is
uniformly distributed on the top of the packing and flows down countercurrent
to the upward air flow. Just above the desiccant inlet is a demister
for ensuring that no air-entrained liquid can pass out of the column
and consists of a 3 in. height of packing rings resting on a wire-grid
support plate. The bottom of the column 5 provides a 4 in. liquid
sump and prevents air from entering the desiccant outflow.
The air/desiccant heat exchanger 19 uses a standard fin/tube core
and is mounted integral with the air blower 3. The unit 19 is used
for fire control of air temperature during closed cycle drying when
the valves 8 and 18 are closed and the valves 7 and 20 are opened,
and to heat ambient air during open cycle drying with valves 7 and
20 closed and valves 8 and 18 opened, during which hot regenerated
desiccant at a temperature of about 142.5.degree. F., for example,
is available from the regeneration loop unit 10 which then passes
through the valve 8 through the unit 19 in the air drying loop
and then back to the storage tank 16.
The desiccant feed pump 17 supplies desiccant alternately to both
the absorber column 5 and to the regeneration column. It is a self-priming
variable speed gear-type pump with Teflon gears and 316 stainless
steel body and shaft, for example. Maximum flow is 1.5 gpm.
The desiccant storage tank 16 is a 10-gallon cylindrical vessel
of Nalgene plastic, for example. Desiccant is withdrawn from the
tank 16 through a Tygon suction line with a filter. Desiccant circulation
through the regeneration column 14 is about 30 pounds per hour at
a temperature of 240.degree. F., for example, and continues during
the open cycle drying in the above fashion until the desired amount
of water has been removed from the desiccant and the hot regenerated
desiccant is ready to be used in the drying loop for closed cycle
crop drying as described hereinabove.
The regeneration column 14 is fabricated from 2.9 in. i.d. copper
tubing and contains 36 in. of packing consisting of 0.28 in. o.d.
by 0.28 in. long glass Raschig rings, for example. A three inch
thickness of this packing is also used as a demister located just
above the desiccant distributor tube. The column 14 is insulated
with 2 in. thick calcium silicate material for minimum heat loss.
Overall column height is about 54 in., for example. The outlet temperature
of the desiccant flowing from the regeneration column 14 is about
180.degree. F., for example.
The condenser 9 is a conventional shell/tube configuration with
the desiccant flowing upward in the shell and the air/water-vapor
mixture and condensate flowing downward through 27 copper tubes
of 0.375 in. o.d. soldered at the ends. The condenser assembly is
also insulated with 2 in. of calcium silicate material.
The blower 22 which circulates air flow between the regeneration
column 14 and the condenser 9 as mentioned above, is a small (2
in. impeller) squirrel cage unit and is driven by a variable speed
ac motor.
The heat exchanger 10 is a counter flow arrangement consisting
of 16 ft. of 0.25 in. o.d. copper tubing within 0.375 in. i.d. Tygon
tubing and formed into a 5 in. diameter coil, for example. The coil
is mounted within a 6.times.6 in. plastic enclosure and surrounded
by fiberglass insulation. Hot desiccant from the bottom of the regeneration
column 14 flows through the annular space between the copper and
Tygon tubing.
The desiccant pump 15 used to pump hot desiccant to the drying
loop heat exchanger 19 is a small oscillating type pump of 0.1
gpm capacity and driven by 60 Hz voltage pulses.
The electrical heater 13 assembly consists of a Chromalox 750 W
copper sheathed immersion heated mounted in a 1.25 in. diameter
copper enclosure. The heater is wrapped with 0.060 in. o.d. copper
wire to increase the heat transfer area and minimize vapor bubble
formation. Insulation for the unit 13 is 1 in. fiberglass covered
with aluminum foil.
The desiccant piping (both loops) is primarily 0.25 in. o.d. copper
tubing with brass compression fittings, for example.
The solar collector 12 is comprised of 12 individual collector
panels arranged in two segments of six panels each. Each panel is
an available commercial design having a selectively coated copper
absorber plate and two layers of cover glass. The collector is used
to furnish a maximum of 44% of the required daily energy input to
the regeneration loop, with the majority of the input furnished
by the electrical heat source 13. The desiccant is circulated directly
through the collector panels without need for a separate transport
fluid and associated heat exchanger. For this reason, copper flow
passages are a necessity to minimize corrosion effects. To obtain
good flow velocity of the desiccant through the collector panels,
a parallel/series arrangement is used in which the desiccant first
flows through a segment of six parallel panels and then through
a second segment of six panels.
In the operation of the system of FIG. 4 to provide for drying
of a 1 bu.test crop (1.25 cu. ft. of raw unshelled peanuts, for
example) using a liquid chloride desiccant, an 18 hour drying cycle
is utilized. Starting with dilute desiccant and a moist crop, the
desiccant is regenerated for 6 hours using the recovered thermal
energy therefrom in the heat exchanger 19 for open-cycle drying
as discussed hereinabove. The drying cycle is then completed during
the next 12 hours of closed-cycle operation, as discussed above,
in which the remaining water in the crop is absorbed into the desiccant
in the column 5. The total water removed from the crop in 18 hours
is about 7.07 pounds.
FIG. 5 of the drawings illustrates a modification of the system
of FIG. 4 utilizing continuous regeneration of the desiccant wherein
grain is initially dried and preheated in a first bin 41 of a two-stage
column dryer in a conventional open loop fashion using heat rejected
in cooling the hot, regenerated desiccant in a heat exchanger 38
which desiccant is fed to the unit 38 from a heat exchanger 30 during
a regeneration of dilute desiccant in the same regeneration manner
as in the system of FIG. 4. It should be understood that a blower,
not shown, is provided for blowing air through the heat exchanger
38 for the heating and drying thereof before passing through the
crop bin 41. The grain then passes into the closed loop portion
of the drying scheme, wherein the air flowing therethrough by means
of a blower 23 passes through an absorption column 25 where the
air is dehumidified and then it is heated in a heat exchanger 39
before passing through a bin 40 of the two-stage column dryer. The
components 25 26 29-37 and 39 of FIG. 5 operate and function
the same as the respective components 5 6 9-17 and 19 of FIG.
4 as already described hereinabove. However, additional energy savings
are achieved because the thermal energy required to preheat the
grain in open loop drying is retained in the closed loop portion
as the grain moves continuously from one stage to the next.
FIGS. 2a and 2b illustrate schematically the closed cycle drying
operation and the absorption loop operation of the system of FIG.
4. FIG. 2a is for the closed cycle drying and FIG. 2b shows the
corresponding changes in the desiccant concentration and temperature.
In the drying bin 1 the drying air picks up the moisture from the
crop and is humidified (i.e., A.fwdarw.B in the diagram). The drying
process requires energy to drive the moisture from the crop, and
this energy is provided by the drying air. Therefore this humidification
process follows the adiabatic cooling path on the psychrometric
chart. The dehumidification process (i.e., B.fwdarw.C) is an exothermic
process, accomplished in the absorption column 5 and hence the
air is heated to some extent but not quite back to the drying temperature.
Although the absolute humidity is the same for C and A, the air
at C must be heated (C.fwdarw.A) to suppress the relative humidity
and thereby the cycle is completed. The required energy for this
heating is provided by the dilute hot desiccant from the column
5 (i.e., process (b) in FIG. 2b). The process (b) is a cooling process
for the used desiccant. The desiccant is heated in the absorption
process (a) from T.sub.1 (90.degree. F.) to T.sub.2 (100.degree.
F.) and the excess sensible energy is used for drying. The absorption
process is a transient one due to continuous change in the concentration
of the desiccant to the column. C.sub.1 is the initial concentration
and C.sub.2 is the final one when the closed cycle drying operation
is completed. The cycle (a')-(b')-(c') represents an intermediate
condition when the tank concentration is at C.sub.3.
FIGS. 3a and 3b illustrate schematically the operation of the open
cycle drying and the regeneration operation, respectively, for the
system of FIG. 5. The starting condition (D) in FIG. 3b for the
regeneration is typically 37% at 90.degree. F. The process (d) is
a heating process to elevate the desiccant temperature to the regeneration
temperature, T.sub.4 and is accomplished in three steps: (1) heating
from T.sub.1 to T.sub.2 is done in the condenser 29 by recovering
heat of varporization; (2) the desiccant is further preheated from
T.sub.2 to T.sub.3 in the heat exchanger 30 using the hot regenerated
desiccant stream from the stripping column 34 (T.sub.3 .fwdarw.T.sub.2
portion of cooling process (f) in the diagram); and (3) the solar
collector 32 and the conventional heater 33 are used to heat it
from T.sub.3 to T.sub.4. The thermal energy in this process (T.sub.3
.fwdarw.T.sub.4) is used in the stripping process (e) to drive out
the moisture from the dilute hot desiccant, which results in cooling
of the desiccant from T.sub.4 to T.sub.3. To complete the cycle
and to use the regenerated desiccant again as a coolant in the condenser
29 until the tank concentration reaches to the desired level, the
temperature of the regenerated stream must be suppressed to the
tank temperature, T.sub.1. This is performed in two steps: (1) the
desiccant is cooled to T.sub.2 in the heat exchanger 30 to preheat
the dilute desiccant (process (d) T.sub.2 .fwdarw.T.sub.3) as described
earlier, and (2) the desiccant is cooled from T.sub.2 to T.sub.1
which is accomplished in the heat exchanger 38 to provide dry air
for the open cycle drying. The net result is that the energy input
to the regeneration system (heating T.sub.3 to T.sub.4 in (d)) is
recovered and is available as the energy output of the regeneration
loop in the form of lower quality energy (cooling T.sub.2 to T.sub.1
in (f)), which can be readily used as an energy source for the open
cycle drying. Due to the transient characteristic of the stripping
column 34 operation, the cycle is repeated until the tank concentration
reaches to C.sub.1. The cycle (d')-(e')-(f')-(g') represents an
intermediate regeneration cycle. In FIG. 3a, the ambient air at
E(65.degree. F., 85% R.H., for example) is introduced to the heat
exchanger 38 and heated to suppress the relative humidity using
the available sensible energy from the regenerated desiccant. The
process F to G represents the drying process. In this process, the
air is humidified, which follows the adiabatic cooling path on the
psychrometric chart, and is expended to the atmosphere at almost
saturated condition. The process E'-F'-G' is basically the same
as the E-F-G path except that the ambient temperature at E' is higher
than that of E.
It should be understood that the system of FIG. 5 could be modified,
if desired, to utilize two separate drying bins each containing
a crop to be dried with one bin in the closed loop air line with
the absorption column and the other bin located in the open loop
air line, and that a separate desiccant tank could be provided with
such a tank coupled between the desiccant outlet from the absorption
column and the condenser of the regeneration loop. With such an
arrangement, the regenerated hot desiccant can then be pumped through
the heat exchanger 39 in the closed loop air line, then through
the heat exchanger 38 in the open loop air line to the absorption
column, and thence to the separate desiccant tank. Thus, two crops
can be dried at the same time with such a modified system.
A third system, not illustrated, for using liquid desiccants combines
crop drying with some other thermal energy use. A good example is
the processing of soybeans. Soybean processing facilities currently
use large amounts of energy both for drying and processing the beans
into oil and high protein meal. In such a system, the process thermal
energy used for tempering, cooking and evaporating hexane (used
to separate the oil from the meal) could be obtained from the thermal
energy available in the regeneration of the desiccant in a manner
similar to the system of FIG. 4 or FIG. 5. The net result of this
arrangement is that drying and processing of the soybeans can be
accomplished with about 14% of the conventional energy requirements
for soybean drying.
The results of three process analyses using liquid desiccants (CaCl.sub.2
or LiCl solutions) are shown in the following table compared with
conventional open cycle dryers.
TABLE __________________________________________________________________________
COMPARISON OF DIFFERENT DRYING SCHEMES Frac. of Energy Btu/lb H.sub.2
O used Compared Case/Description Removed with Conventional __________________________________________________________________________
Desiccant - 1 crop drying bin 1100 0.50 (2 mode drying) Desiccant
- 2 stage drying 1000 0.45 (single bin or continuous) Desiccant
- 1 stage drying 300 0.14 with non-drying energy use Conventional
- Open cycle 2210 1.00 crop drying __________________________________________________________________________
The three desiccant cases represent three different ways of using
the energy recovered during the regeneration cycle, namely: 1 crop
bin, two-mode drying (FIG. 4 or FIG. 6 to be described hereinbelow),
two stage drying (FIG. 5), and alternate non-drying energy use for
rejected thermal energy. The results shown are in the form of the
total energy required to remove 1 pound of water from a crop (peanuts
in this case). Both thermal energy and electrical energy (for pumps
and blowers) are included in the totals.
FIG. 6 illustrates still another embodiment of the present invention
for crop drying in which a liquid desiccant is utilized. The system
of FIG. 6 operates in substantially the same manner as the system
of FIG. 4 with the exception that an evaporator is utilized in
the desiccant regeneration loop instead of a stripping column. During
the regeneration of the dilute desiccant, the system of FIG. 6 is
operated in the open cycle drying mode (first six hours) wherein
the valves 47 and 60 are closed, the air valves 42 and 62 are opened,
the air valve 44 is closed, and ambient air is blown through a crop
bin 61 by means of a blower 43 after passing through a heat exchanger
59 which is in the regeneration loop at this time. During the open
cycle drying mode, dilute desiccant is pumped from its storage tank
56 by means of a pump 57 through a valve 58 to a condenser 49 and
thence to a heat exchanger 50. The dilute desiccant is preheated
in the units 49 and 50 before it is passed through a heater 53 (solar
or fossil or a combination thereof). The finally heated dilute desiccant
is then fed to an evaporator 54 where the water vapor in the desiccant
is driven off and is then condensed in the condenser 49 before passing
to a drain through a valve 55. The now hot, dry regenerated desiccant
then passes through the heat exchanger 50 through a valve 48 and
the heat exchanger 59 (utilized for the open-cycle drying operation)
and then back to the storage tank 56. After the desiccant has been
completely regenerated, then the valves 48 and 58 are closed, the
valves 47 and 60 are then opened, the air valves 42 and 62 are closed,
the air valve 44 is opened, and then for the next 12 hours, the
desiccant from the tank 56 is pumped by the pump 57 through the
valve 60 to the absorption column 45 and thence through the valve
47 and through the heat exchanger 59 back to the tank 56 in a continuous
manner, thus constituting the closed-cycle portion of the complete
drying cycle. It should be noted that the non-condensables are removed
from the condenser 49 by means of an evacuating means, not shown,
by way of a valve 51.
Analysis of the chemical and phsyical properties of desiccants
indicates that calcium chloride and lithium chloride salt solutions,
as discussed hereinabove, have the best application in solar regenerated
desiccant crop drying apparatus. These solutions can be regenerated
at lower temperatures than other desiccants (which relaxes constraints
on the choice of solar collector hardware), have high specific heats
(which augments thermal storage capacity), and are stable, nontoxic
chemicals.
The present invention has been described by utilizing a liquid
desiccant in the respective embodiments thereof. However, it should
be understood that a solid desiccant (silica gel, for example) could
be utilized if such were desired. In such a system two beds containing
a solid desiccant, and a drying bin could be provided wherein one
desiccant bed is coupled to the drying bin in a closed loop fashion
while at the same time the other desiccant bed is being regenerated
after which the regenerated bed is coupled to the drying bin and
the first bed can then be regenerated, etc.
This invention has been described by way of illustration rather
than by limitation and it should be apparent that it is equally
applicable in fields other than those described.
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