Molecular sieve abstract
A fluid drying system comprising a molecular sieve bed of crystalline
metal aluminosilicate zeolite particles for removing water from
a moisture-laden process stream. The improved apparatus and methods
provides means for measuring electrical conductivity of the zeolite
particles and generating a signal representative of sorbed water
content. Means responsive to the signal is provided for interrupting
process fluid flow through the bed at a predetermined bed water
content. In the preferred embodiments a high voltage is imposed
across the bed for regenerating the bed to separate sorbed water
from the zeolite particles.
Molecular sieve claims
What is claimed is:
1. A gas drying system comprising:
a. an absorbent bed of zeolite particles;
b. a plurality of spaced apart electrodes in contact with the bed;
c. means for measuring electrical current flow through the bed
between the spaced electrodes and generating a signal representative
of adsorbed water content;
d. means responsive to signal for interrupting gas flow through
the bed at a predetermined bed water content; and;
e. means for applying electrical energy to the bed between the
spaced electrodes to regenerate the bed, said electrical energy
being provided by said means at a voltage gradient of about 0.05
to 500 Kv/cm, a current density of about 0.001 microamps to 1 amp/cm.sup.2
and a frequency of 0 to 10.sup.3 Hz.
2. The system of claim 1 wherein means are provided to apply vacuum
to the bed during regeneration.
3. The system of claim 1 wherein means are provided to pass dry
purge gas through the bed during regeneration.
4. The system of claim 1 wherein said means for applying electrical
energy provides a voltage gradient of about 0.2 to 10 Kv/cm and
a current density of 0.01 to 100 microamps/cm.sup.2.
5. A process for drying a gas stream which comprises:
a. passing a moisture containing gas stream through a bed of zeolite
particles to adsorb water from said gas;
b. measuring electrical conductivity of the zeolite bed to determine
the content of adsorbed water therein;
c. interrupting said gas stream when the water content of said
bed has reached a predetermined level; and
d. regenerating said bed to remove adsorbed water by application
of electrical energy to said bed, said electrical energy being applied
at a voltage gradient of about 0.05 to 500 Kv/cm, a current density
of about 0.001 microamps to 1 amp/cm.sup.2 and a frequency of about
0 to 10.sup.3 Hz.
6. The process of claim 5 wherein vacuum is applied to said bed
during regeneration.
7. The method of claim 5 wherein purge gas is passed through said
bed during regeneration.
8. The method of claim 5 wherein subsequent to the regeneration
step the process is repeated.
9. The process of claim 5 wherein the frequency is 50 to 60 Hz.
10. The process of claim 5 wherein said gas is air.
11. The process of claim 5 wherein said current density is about
0.01 to 100 microamps/cm.sup.2.
12. The process of claim 5 where said voltage gradient is about
0.5 to 2 Kv/cm.
13. The process of claim 5 wherein said electrical energy is applied
at a voltage gradient of about 0.2 to 10 Kv/cm and a current density
of about 0.01 to 100 microamps/cm.sup.2.
Molecular sieve description
This invention relates to molecular sieve drying systems. In particular,
it relates to methods and apparatus for removing moisture from fluid
streams and for regenerating or reactivating moisture-laden zeolite
particles of the alkali metal aluminosilicate type. Molecular sieves
made from natural or synthetic crystalline alkali-metal alumino-silicates
of the zeolite type have been found useful for removing selected
components from fluid streams. Drying of fluids such as air, petroleum
feedstocks or industrial gases has provided a substantial use for
molecular sieve.
Gases can be dried to a water content of a few parts per million.
In many systems designed to condition feed to cryogenic plants,
the gas must be dried to a fraction of a part per million-- low
enough to make deriming of heat exchangers a very rare necessity,
even when the gas is taken all the way to the liquid phase. This
super-drying can be accomplished even when the feed gas is at high
temperature because the dewpoints of molecular sieve dehydration
are not a function of inlet temperature, and because these unique
adsorbents maintain high capacity even when operating at high temperatures,
The ability to handle high temperature feed while producing completely
dry gas is a unique characteristic of molecular sieve systems. In
addition, the performance of molecular sieves is not affected by
the degree of saturation of the feed.
The problem of drying large volumes of fluids at a rapid rate is
particularly pressing in the operation of petroleum refineries in
which large quantities of hydrocarbon fluids are handled daily.
The increase in the yield of product which accompanies such reduction
in the water content of the charging stock in many instances more
than compensates for the cost of drying the charging stock with
chemical drying agents. Although the problem of drying hydrocarbon
fluids on a continuous basis is a typical large scale application
of the present process because of the large volumes of the hydrocarbon
streams utilized in the petroleum industry, the process may be used
in many fluid streams (whether normally liquid or gaseous) which
are essentially non-reactive with the particular desiccant involved
in the process. Thus, moist streams such as air nitrogen, carbon
monoxide, carbon dioxide, halogenated hydrocarbon chlorobenzene,
and others are nonreactive with approppriate inorganic desiccants
and may be utilized as feed stocks for molecular sieve drying processes.
The sieves are inert to most process fluids and physically stable
in normal bed depths even when wet with water.
The desiccant properties of molecular sieves are carried to higher
temperatures than those of other adsorbents. Typical capacity is
16.5% at 95.degree. C. and 4% even at 230.degree. C. The amount
of water adsorbed has little effect on their drying efficiency up
to the "break point" (the point where the vapor pressure
increases abruptly). Dewpoints below -75.degree. C., even with gases
as high as 100.degree. C., may be realized. Molecular sieves dry
gases at high superficial velocities even with low relative humidity
feed gases. The velocity usually ranges from 10 to 50 m/min with
zeolite agents. For drying purposes, smaller-pore-size molecular
sieves (3 A) are often employed to reduce coadsorption of other
materials.
Synthetic crystalline alkali-metal alumino-silicates of the faujasite
type are described in U.S. Pat. Nos. 2882243 2882244 incorporated
herein by reference.
PRIOR ART REGENERATION METHODS
The exhausted bed must be regenerated to remove the adsorbate in
preparation for the next adsorption step. Normally, the main flow
will be switched to a second adsorption tower during this regeneration
to provide a continuous operation. In the prior art, regeneration
may be accomplished in several ways, the choice depending on technical
and economic considerations. Regeneration methods in the past have
depended on the same principle-- the process conditions surrounding
the adsorbent are changed to those corresponding to a very low equilibrium
capacity. In general, the greater the difference between the equilibrium
capacities of adsorption and regeneration, the more rapid and complete
the regeneration.
In typical cyclic systems, the adsorbate is removed from molecular
sieve beds by heating and purging with a carrier gas. This regenerates
the adsorbent and prepares it for the next adsorption cycle. During
regeneration, sufficient heat must be available to raise the temperature
of the adsorbent, the adsorbate, and the vessel, plus an additional
amount to vaporize the liquid and offset the heat of wetting of
the molecular sieve surface. In most practical designs, gas temperatures
in excess of the adsorbate's boiling point are used to increase
the rate of heat input to the system. When regeneration temperatures
are considered, it is the bed temperature (the temperature of the
molecular sieve beads) that is critical. Bed temperatures in the
200.degree. to 300.degree. C. range are usually employed.
After regeneration, a cooling period reduces the molecular sieve
temperature to about 15-20.degree. C. above the temperature of the
stream being processed. This is most conveniently done by using
the same gas stream as for heating, but with no heat input. The
thermal method involves heating to a temperature at which the adsorptive
capacity is reduced to a low level so that the adsorbate leaves
the molecular sieve surface and is easily removed by a small stream
of purge gas. This can be done at operating pressure, or at a reduced
pressure.
The "pressure swing" regeneration method similarly depends
on reducing the adsorptive capacity by lowering the pressure at
essentially constant temperature.
In another method the adsorbate is removed without changing the
temperature or pressure, by passage of a fluid (liquid or gas) containing
no adsorbable molecules, and in which the adsorbate is soluble or
miscible.
Changing the temperature or pressure by passing of a fluid containing
a high concentration of an adsorbable molecule can also effect desorption.
Because of this high concentration, these molecules are able to
displace material previously adsorbed. In the case of liquids, the
resulting mixture is then separated, by distillation, into a saleable
product of high purity and the regenerating fluid (which is reused).
Regeneration of a wet molecular sieve bed by electrolysis is disclosed
in U.S. Pat. No. 3474023 by application of a low DC potential
to evolve H.sub.2 and O.sub.2. Also, application of high frequency
electrical energy to effect dielectric heating of the bed particles
is shown in U.S. Pat. No. 3359707.
It is known that the heat of adsorption of water is aproximately
the same as the heat of vaporization. Most regeneration methods
consume far in excess of this to remove the adsorbate. In view of
the widespread use of molecular sieve adsorption, especially in
drying air, hydrocarbon feedstocks and industrial gases, there is
a definite need for a sieve regeneration process that is fast, economical
and easily controlled.
SUMMARY OF THE INVENTION
A novel regeneration system for a molecular sieve fluid drier has
been discovered within moisture is sorbed from a fluid stream by
a packed bed of zeolite particles, where it is detected electrically.
The system provides methods and means for measuring electrical conductivity
of the packed bed and generating a signal representative of moisture
content of the bed; comparing the signal with a predetermined value;
interrupting the fluid stream through the bed when the predetermined
value is exceeded; recharging the bed by removing sufficient sorbed
water to regenerate the bed; and reinitiating flow of the fluid
stream through the bed.
In the preferred embodiment the system includes a plurality of
spaced-apart electrodes in contact with the bed; a source of high
voltage electrical energy; and means for measuring electrical current
flow through the bed between the spaced electrodes. As part of an
overall drying system the invention further has means for applying
the high voltage energy to the bed between the spaced electrodes
to regenerate the bed, and a vacuum or purge gas may be applied
to the bed concurrently with the high voltage electrical energy.
The system is useful for zeolite particles, such as Type A, Type
L, Type X or Type Y zeolites, having an average particle size of
at least 1 .mu., and is especially adapted for use with a regeneration
sub-system which comprises spaced-apart electrically conducting
members having the molecular sieve bed substantially there-between;
and electrical means for imposing a recharging voltage of about
0.2 Kv/cm to 10 Kv/cm between the electrically conducting members.
The invention has as an important object a system for gas drying
comprising a porous molecular sieve bed comprising crystalline metal
alumino-silicate zeolite particles for removing water from a moisture
laden gas stream, including means for measuring electrical conductivity
of the zeolite particles and generating a signal representative
of sorbed water content; means responsive to said signal for interrupting
gas flow through the bed at a predetermined bed water content before
saturation; and means for regenerating the bed to separate sorbed
water from the zeolite particles.
These and other objects and features of the invention will be apparent
to a skilled scientist by reference to the following description
and in the drawing.
THE DRAWING
FIG. 1 is a sideview, partially cut away, of typical electro-desorption
apparatus, according to the present invention;
FIGS. 2 and 3 are schematic representations of alternative embodiments;
and
FIG. 4 is a graphic plot of electrical resistivity and moisture
content for a molecular sieve bed.
DESCRIPTION
The molecular sieve materials consist essentially of crystalline,
hydrated metal aluminosilicates with a number of unusual properties.
The most important types of molecular sieves are made synthetically,
but their structure is similar enough to certain naturally occurring
minerals to be classified as zeolites. Although the crystal structures
of some of the molecular sieves are quite different (two types,
A and X are most important), their significance as commercial adsorbents
depends on the fact that in each the crystals contain interconnecting
cavities of uniform size, separated by narrower openings, or pores,
of equal uniformity. When formed, this crystalline network is full
of water, but with moderate heating, the moisture can be driven
from the cavities without changing the crystalline structure. This
leaves the cavities with their combined surface area and pore volume
available for adsorption of water or other materials. The process
of evacuation and refilling the cavities may be repeated indefinitely,
under favorable conditions.
With molecular sieves close process control is possible because
the pores of the crystalline network are uniform rather than of
varied dimensions, as is the case with other adsorbents. With this
large surface area and pore volume, molecular sieves can make separations
of molecules, utilizing pore uniformity, to differentiate on the
basis of molecular size and configuration.
Molecular sieves are crystalline, metal aluminosilicates with three
dimensional network structures of silica and alumina tetrahedra.
This very uniform crystalline structure imparts to the Molecular
Sieves properties which make them excellent desiccants, with a high
capacity even at elevated temperatures. Some molecular sieves, in
addition to this high adsorptive capacity, have the ability to indicate
relative humidity by a change in color, which can be utilized to
determine the point where reactivation is required.
The crystalline metal alumino-silicates have a three-dimensional
interconnecting network structure of silica and alumina tetrahedra.
The tetrahedra are formed by four oxygen atoms surrounding a silicon
or aluminum atom. Each oxygen has two negative charges and each
silicon has four positive charges. This structure permits a sharing
arrangement, building tetrahedra uniformly in four directions. The
trivalency of aluminum causes the alumina tetrahedron to be negatively
charged, requiring an additional cation to balance the system. Thus,
the final structure has sodium, potassium, calcium or other cations
in the network. These charge balancing cations are the exchangeable
ions of the zeolite structure.
In the crystalline structure, up to half of the quadrivalent silicon
atoms can be replaced by trivalent aluminum atoms. Zeolites containing
different ratios of silicon to aluminum ions are available, as well
as different crystal structures containing various cations.
In the most common commercial zeolite, Type A, the tetrahedra are
grouped to form a truncated octahedron with a silica or alumina
tetrahedron at each point. This structure is known as a sodalite
cage.
When sodalite cages are stacked in simple cubic forms, the result
is a network of cavities approximately 11.5A in size, accessible
through openings on all six sides. These openings are surrounded
by eight oxygen ions. One or more exchangeable cations also partially
block the face area. In the sodium form, this ring of oxygen ions
provides an opening of 4.2A in diameter into the interior of the
structure. This crystalline structure is represented chemically
by the following formula:
the water of hydration which fills the cavities during crystallization
is loosely bound and can be removed by moderate heating. The voids
formerly occupied by this water can be refilled by adsorbing a variety
of gases and liquids. The number of water molecules in the structure
(the value of X) can be as great as 27.
The sodium ions which are associated with the aluminum tetrahedra,
tend to block the openings, or conversely may assist the passage
of slightly oversized molecules by their electrical charge. As a
result, this sodium form of the molecular sieve, which is commercially
called 4A, can be regarded as having uniform openings of approximately
4A diameter.
Because of their base exchange properties, zeolites can be readily
produced with other metals substituting for a portion of the sodium.
Among the synthetic zeolites, two modifications have been found
particularly useful in industry. By replacing a large fraction of
the sodium with potassium ions, the 3A molecular sieve is formed
(with openings of about 3A). Similarly, when calcium ions are used
for exchange, the 5A (with approximately 5A openings) is formed.
The crystal structure of the Type X zeolite is built up by arranging
the basic sodalite cages in a tetrahedral stacking (diamond structure)
with bridging across the six-membered oxygen atom ring. These rings
provide opening 9-10A in diameter into the interior of the structure.
The overall electrical charge is balanced by positively charged
cation(s), as in the Type A structure. The chemical formula that
represents the unit cell of Type X molecular sieve in the soda form
is shown below:
as in the case of the Type A crystals, water of hydration can be
removed by moderate heating and the voids thus created can be refilled
with other liquids or gases. The value of X can be as great as 276.
A prime requisite for any adsorbent is the possession of a large
surface area per unit volume. In addition, the surface must be chemically
inert and available to the required adsorbate(s). From a purely
theoretical point of view, the rate at which molecules may be adsorbed,
other factors being equal, will depend on the rate at which they
contact the surface of adsorbent particles and the speed with which
they diffuse into particles after contact. One or the other of these
factors may be controlling in any given situation. One way to speed
the mass transfer, in either case, is to reduce the size of the
adsorbent particles.
While the synthetic crystals of zeolites are relatively small,
e.g., 0.1 .mu. to 10 .mu., these smaller particles may be bonded
or agglomerated into larger shapes. Typical commercial spherical
particles have an average bonded particle size of 100 .mu. to 500
.mu. (4 .times. 12 mesh). Other molecular sieve shapes, such as
pellets (1-3 mm diameter), Rashig rings, saddles, etc., are useful
for continuous sorption processes. The preferred molecular sieve
materials are Type A, L, X and Y zeolites or mixtures of these zeolites,
having an average particle size of about 1 .mu. to 10 .mu. for powder
or 100 .mu. to 500 .mu. for bonded particles.
Referring to FIG. 1 of the drawing, a fluid drying apparatus 10
is shown partially cut away. A vertical cylindrical vessel 12 provides
a drying chamber. Fluid to be dried is introduced to chamber 12
through fluid inlet means comprising conduit 14 and T-connection
15. Screen 18 is supported at the lower end of vessel 12 by annular
ring 19. Screen 18 may be fabricated of metal or suitable material
having sufficient strength to support a bed of dielectric absorbent
particles 20 such as zeolite molecular sieve particles. A concentric
metal electrode 24 is inserted through vessel 12 in contact with
particles 20. Electrode 24 is held in fixed position by electrically
insulated bushing 26 connected to T-connection 17. Electrode 24
is operatively connected to power source 40 by electrical lead 41.
The power source is connected to ground by electrical lead 42.
Vessel 12 may be constructed of an electrically conducted material
such as steel to provide an electrical path for direct contact with
particulate bed 20. Vessel 12 may be connected to ground by electrical
lead 44. Means for draining the vessel 12 may be provided by fluid
conduit 28 having valve 29 disposed therein. Conduit 14 is provided
with means, such as a control valve, for interrupting inlet fluid
flow during regeneration. Discharge outlet 16 can be connected alternatively
to a downstream utilization or vented to atmosphere during regeneration
to remove sorbate vapor.
The electrodesorption regeneration method may be used with a dry
purge gas passing through the bed during regeneration, or a vacuum
can be maintained by suitable pressure seals and valving of the
system.
The drying cell configuration may be adapted to different process
requirements. High gas throughput is obtainable for many processes.
Condensation of water vapor or desorbed liquid may require a gravity
liquid flow through the particulate bed to a drainport, as shown
in FIG. 1. In other systems, the electrodesorbed component is removed
only in the vapor phase.
The vessel 12 may be constructed of electrically insulating material
such as polyvinyl chloride (PVC), nylon phenolic, acrylic, or ABS
resin, glass, glass-lined steel, or wound fiberglass/resin. Where
a case electrode is employed, the shell may be metal or metal-lined.
Electrodes may be constructed of sintered metal powder, steel wool,
drilled carbon or other foraminous electrically conducting materials.
Powdered sieve may be contained by porous metal screen/wool electrode
structures.
The physical state of the bed while drying a fluid need not be
the same as during the electrodesorption step. The degree of compaction
can vary widely within the operable limits of the system. During
regeneration, the zeolite particles should be maintained in a physical
state to permit electrical flow from a first electrode to a second
electrode through an electrical path from particle to particle.
Ordinarily, a void volume of less than 50 vol% is suitable to achieve
this condition. Loosely-packed fluid-permeable molecular sieve beds
have a macro-porosity or void volume of about 30-vol%. It is believed
that the flow of electrical current takes place on the particle
surface due to mobility of the alkali metal ion in the sorbed water
phase.
In a preferred practice of my invention the adsorbent bed is regenerated
using the electrodesorbtion technique set forth in my copending
application Ser. No. 625237 filed Oct. 23 1975. As set forth in
that application, the adsorbent bed is economically and rapidly
regenerated by application of electrical energy to the bed at a
preferred voltage of about 0.05 to 500 kv/cm, a current density
of about 0.01 to 100 microamps/cm.sup.2 and a frequency of about
0 to 10.sup.3 Hz.
A comparison of my preferred electrodesorbtion method of regeneration
with a typical prior art method of regeneration such as thermal
reactivation reveals the efficient use of energy achieved by electrodesorbtion.
For example, the heat of adsorbtion and desorbtion of water as vapor
on Type A sodium molecular sieve (NaA) is about 1000 cal/g of H.sub.2
O at 20.degree. C. The theoretical heat of desorbtion when water
is desorbed in the liquid phase may be calculated as the heat of
desorbtion less the heat of vaporization of water that is 1000 less
540 or about 460 cal/g. Accordingly, if water is desorbed from the
molecular sieve mostly in the form of liquid water, the minimum
heat for regeneration can theoretically approach 460 cal/g. However,
it is observed that the energy required to electrodesorb molecular
water from NaA at a reasonable rate is about 1 to 2 times the heat
of desorbtion of water as vapor, that is, 1000 to 2000 cal/g H .sub.2
O.
In contrast, the amount of heat required to desorb water via a
conventional heating process as vapor from NaA is equal to the heat
of desorbtion, plus the heat of vaporization, plus the heat required
to the adsorbent bed and desorbed water vapor to an elevated regeneration
temperature at which desorbtion will occur at a reasonable rate.
A typical adsorbtion column having a diameter of 183 cm and a height
of 305 cm and a volume of 8.02 .times. 10.sup.6 cm.sup.3 will contain
5.53 .times. 10.sup.6 g of NaA. It is desired to dry in air to a
dew point of +10.degree. C. or 3000 ppm H.sub.2 O. It is assumed
that to obtain air of this degree of dryness, the bed is placed
on an adsorbtion cycle until 5% of its saturated capacity is achieved.
Therefore, the amount of H.sub.2 O on the bed is equal to:
If the heat lost to radiation and convection during a typical 2
to 4 hour regeneration cycle is neglected, the amount of heat required
to heat the NaA in the bed to 230.degree. C. from 20.degree. is
equal to:
The heat of desorbtion as water vapor is equal to:
The heat necessary to raise the temperature of the water vapor
to the regeneration temperature is equal to:
The total heat required for conventional regeneraton of the bed
is the sum of items (2), (3) and (4) above or about (5) 3.6 .times.
10.sup.8 cal.
In contrast to the above, it is found that the heat required to
regenerate the same bed using the electrodesorbtion method is about
one to two times the heat of adsorbption as liquid water. Assuming
the amount of energy required by my process to be twice the heat
of adsorption of water vapor, the amount of heat required to regenerate
the bed by electrodesorbtion is equal to:
Comparing the value (5) above with (6), the energy saving advantage
obtained using electrodesorbtion may be expressed as follows:
That is, electrdesorbtion utilizes about one-third the heat required
to regenerate a molecular sieve drying bed using a standard thermal
reactivation technique.
From the above it is seen that from an energy standpoint electrodesorbtion
is considerably more efficient than conventional thermal methods.
However, more importantly, it is observed that the time required
to achieve regeneration using electrodesorbtion is several times
less than that required by thermal means. For example, in the bed
described above it is found that about 3 hours are required to achieve
satisfactory regeneration at 230.degree. C. Using electrodesorbtion
it is estimated that satisfactory regeneration may be obtained in
as little as 3 minutes. These rapid regeneration times permit the
use of much smaller beds in that the size of the beds is no longer
governed by lengthy reactivation times.
While the mechanism of electro-desorbtion of wet zeolites is not
completely understood, it is believed that the water is first desorbed
as a liquid and vaporized from a thin electrolyte film by energy
dissipated in the film as heat during application of high voltage.
As water is removed from the particles by desorption and evaporation,
the bulk resistivity increases. As set forth above, at a predetermined
moisture content, as measured by bed conductivity, the regeneration
step is terminated and the drying cycle is continued.
To facilitate a uniform rate removal of water, the regeneration
step may be carried out under controlled conditions, such as constant
current. As sorbed water is removed from the bed, electrical conductivity
decreases, which requires an increase in voltage to maintain the
desired current. The current density is preferably maintained at
a value of about 0.01 to 100 .mu.a/cm.sup.2 (microamperes per square
centimeter), with optimum performance for most zeolites being obtained
under constant current conditions in the range of 1 to 10 .mu.a/cm.sup.2.
Currents as low as 0.001 .mu.a/cm.sup.2 or as high as 1 a/cm.sup.2
are feasible. The above current density values are based on uniformly-sized
electrodes. It is understood that different maximum and minimum
values may be applied to electrically different areas.
For reactivating moisture-loaded metal alumino silicates, the voltage
gradient preferably is about 0.2 to 10 Kv/cm, with best results
being obtained in the 0.5 to 2 Kv/cm range. However, it is possible
to use voltages up to the electrical breakdown of the strongest
dielectric zeolite (up to 500 Kv/cm).
The bulk resistivity (.rho.) of zeolite particles is measured in
a packed bed having the particles in contact with one another and
completely filling the space between uniformly shaped parallel conductors.
The measured resistance (R) is expressed as R=.rho.1/A, where .rho.
is the bulk resistivity (ohms-cm.sup.2 /cm), 1 is the interelectrode
distance (cm) and A is the cell cross-sectional area (cm.sup.2).
Current density is a function of bulk resistivity, applied voltage
and interelectrode distance, according to the equation: ##EQU1##
where I/A = current density (amperes per cm.sup.2)
As moisture content of a sieve bed increases, the conductivity
increases (.rho. decreases) and more current flows for a given field
strength (E/1). In order to maintain constant current during water
removal, the electrical field is increased proportionally to conductivity.
Thus, when batch reactivation is started, a relatively low voltage
gradient is applied and increasing voltage is applied as the water
is driven off. The final voltage may be as high as ten times the
initial value (E.sub.f = 10 .times. E.sub.o).
The power supply may provide a continuous DC potential, pulsed
DC, a square wave or sinusoidal wave of alternating current. Relatively
low frequencies of 0 to 60 Hz are preferred; however, the skin conductance
phenomenon is efficacious at higher frequencies, for instance 400
Hz or as high as 10.sup.3 Hz. Radio frequencies, such as produced
by a HV generator (about 10.sup.7 Hz), cause overheating of the
dielectric sieve adsorbent and are not as efficient in energy consumption
as the preferred lower frequencies. DC and very low frequency (0-60H.sub.z)
power supplies are preferred because of the large power factors
achieved, as compared to HF generators or other relatively high
frequency sources. By employing such electrical supplies, the heating
may be confined largely to electrolytic film or surface layer of
the adsorbent crystallite structure without heating the body of
the adsorbent itself.
While the measuring electrodes may be integral with regenerating
electrodes, a separate set may be provided for each function, as
shown in FIG. 2. Electrodes 12A and 24A are connected to a source
of high voltage alternating current 40A which is energized in response
to a signal derived from a DC current between foraminous screen
measuring electrodes 12B and 24B. The measuring current is parallel
to the fluid flow, while the regenerating current is transverse.
It is understood that these functions may be transposed.
The same source of electrical energy may be employed for both moisture
measurement and regeneration. Since 1/2 to 1 minute or longer is
ordinarily required at full power to reactivate a moisture-laden
bed, a short high voltage pulse may be used for moisture measurements
without substantial heating of the bed. For instance, in one case
tested a source of 20 KV-DC was applied to a molecular sieve bed
during the drying cycle to measure leakage current.
A typical integral electrode system is shown in FIG. 3 wherein
a regeneration type molecular sieve apparatus 10 comprising electrically
insulated vessel 10 enclosed a molecular sieve bed. Spaced electrodes
12 and 24 are connected to power source 40. An ammeter in series
connection measures the electrical current and generates a signal
representative of moisture content in the packed particles between
the electrodes. A control system, shown in block diagram, is responsive
to this signal to operate a valve to interrupt the process stream
during degeneration. Under command from the control system the power
source may be operated to apply full regeneration potential across
the bed through the electrodes sufficient to effect regeneration.
During this part of the cycle a purge gas or vacuum may be applied
to the vessel 12. When the ammeter signal indicates that moisture
has been removed to the desired level, the control system then initiates
the drying cycle and returns the power supply to a sensing mode.
The activation or regeneration cycle may be controlled or terminated
by pre-set timer, dew-point measurements in the outlet purge gas,
bed electrical resistivity signal, regeneration energy consumption,
or otherwise.
The moisture-sensing current may be a continuous wave or intermittent
pulse type of current. Pulse frequency and duration as well as voltage
may be selected to inter-connect the moisture-sensing circuitry
with the other functions of the control system. Sequencing of regeneration
equipment and of fluid handling equipment may be controlled by a
central control module or on - line process computer. These functions
are selected according to the process requirements.
It is understood that certain elements of the apparatus may perform
multiple functions; for instance, the electrodes for sensing bed
moisture may also be employed at high voltage for regenerating the
bed. This may require switching the power supply in a known manner.
The moisture-sensing current may itself provide the signal representative
of moisture content or a transmitting ammeter instrument may convert
this current to a desired electrical, mechanical or fluid signal
to represent the value. The signal is compared with a predetermined
value to initiate the regeneration cycle for periodically removing
water from the molecular sieve bed.
By employing direct low-frequency (0-1000 Hz) electrical energy
for removing the absorbate, regeneration can be effected in a time
span which is a very small fraction of the time elapsed in loading
the molecular sieve during the sorption cycle. Whereas a typical
gas drying system may require more than 2 hours to reach moisture
capacity (break point), the same amount of water may be removed
from the desiccant in about 1 minute or less, which gives a regeneration
time less then 1% of the sorption time.
In FIG. 4 a plot of electrical resistivity vs. moisture content
is given for two different cell configurations. The data for curve
A were obtained using porous screen No. 18 stainless disc electrodes
in a cylindrical acrylic plastic tube with a bed 3.2 mm. in diameter
and 28 cm long. The particles were 4-5A type x Davison Grade 714
zeolite (8 .times. 12 mesh). The air flow through the packed bed
was 53 l/min (STP) with an inlet water content of 2500 ppm and outlet
content of about 2 ppm. The sieve adsorbs about 6.8% H.sub.2 O per
hour under ambient conditions. Electrical measurements between spaced
end electrodes were made using a DC potential of + 18.5 VDC applied
with the amode in the downstream position. The upswing in resistivity
shows the bed approaching moisture saturation, requiring regeneraton.
Curve B shows the relationship center rod-case electrodes as depicted
in FIG. 1.
While the invention has been demonstrated by particular examples,
there is no intent to limit the inventive concept except as set
forth in the following claims.
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