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Description  |
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FIELD OF THE INVENTION
This invention relates to methods and apparatus for constructing and
utilizing a fluid dispensing device, and more particularly, to fluid
devices for generating extremely small flow rates for use as a trace
component for calibration or measurement purposes.
BACKGROUND OF THE INVENTION
Liquid as well as gas permeation devices are utilized to generate extremely
small flow rates by permeating a volatile fluid which is transmitted in a
permeation process by a permeand passing through a permeation barrier and
becoming a permeate upon leaving the barrier. Various permeation materials
include fluoroplastic, acetal, silicone, rubber, neoprene, viton, and
polypropylene. Permeands include benzene, alcohol, water, toluene,
hydrogen sulfide methane and so forth.
Heretofore, obtaining controlled rates of flow from a permeation barrier
for trace fluids has been difficult to obtain and control, and in the
present invention, control of permeation is obtained precisely.
Also, sometime there has been no practical way for analyzing samples of
process gases which have impurities. In the present invention, process
streams with impurities can be cleaned up.
The present invention provides a structure that is suitable at temperatures
near ambient for permeands that are liquid at ambient temperature and
pressure.
A further aspect of the present invention is a system for injecting gas
such as hydrogen sulfide in small rates of flow into a process stream.
Prior patent art known to Applicant include 3,788,545; 3,856,204; and
4,399,942.
SUMMARY OF THE PRESENT INVENTION
In the present invention the permeation member is an elongated member
disposed within an elongated container containing a permeand where the
permeation member has a sized blind bore extending along the length of the
permeation member, it being recognized that a tubular permeation member
will provide many of the same benefits and is an alternate configuration.
The permeation member being within the container, may be under
compression, as opposed to tension, to reduce stress cracking of the
permeation member and increases resistance to rupture resulting from the
pressure of the permeand.
The elongated blind bore has a long length and a small diameter so that a
fringe effect due to diagonal travel of the permeand is diminished
relative to the total permeation area. This provides a more predicable
output rate from a given configuration. The output rate can be easily
changed, even after the container is filled with permeand, by drilling a
blind bore deeper or by increasing the diameter of the blind bore.
If the barrier is allowed to contact gases such as moisture, this can cause
a change in permeation rate. This is prevented by a metal filler rod sized
smaller than the blind bore which is inserted into the blind bore to
reduce the exit and the internal volume opening. An annular space of less
than 0.001 inch can be obtained, minimizing back diffusion. The filler rod
is preferably curved to provide for frictional retention in the blind
bore.
In another aspect of the present invention, if the liquid vapor in the
container is below atmospheric pressure, there is insufficient pressure to
produce a vapor flow of permeate. This is overcome by forcing a carrier
fluid through a dip tube to a location near the bottom of the blind bore
so that carrier fluid is forced into the annulus between the dip tube and
the blind bore so as to reduce the partial pressure of the permeate
sufficiently to allow permeate flow. A flow through arrangement can also
cause the same result as use of a dip tube.
In another aspect of the present invention, a housing means contains a
permeand and a permeation member inside the housing means where the
permeation member is used for regulating permeand flow. A fixed volume
container is used for establishing volume of carrier fluid. The duration
of time the permeable flow is allowed to enter the fixed volume determines
the concentration in the fixed volume. Permeand flow can be set to make
one part per million mixture for each second of flow into the fixed
volume.
In still another aspect of the present invention, a permeand in a gaseous
state can be utilized. For example, nitrogen or carbon monoxide in a gas
state can be fed into and out of the container and provide a gas under
pressure within the container. The permeate output from the permeation
member can be controlled by regulating the pressure of the input gas. If
the permeand gas is not pure, it is necessary to have an output flow of
permeand to sweep out impurities.
In analyzing trace gas in a process stream, the particulates, mists, free
liquids and other components can be restricted by a permeation barrier
while the permeate is transferred into a clean carrier gas. The analyzer
is protected by the permeation member from undesirable components. In the
case of hydrogen sulfide in fuel gas, the reduction in concentration of
the hydrogen sulfide by the carrier gas is of considerable benefit, as
some analyzers operate best at low concentrations.
DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates in longitudinal cross section a container arrangement
for obtaining controlled rates of flow;
FIG. 2 illustrates in longitudinal cross section an arrangement for
introducing fluid such as water vapor into a carrier fluid even though
water has a low vapor pressure; and
FIG. 3 illustrates in longitudinal cross section, an arrangement for
obtaining controlled rates of flow for gases which can not easily be
liquified, or to eliminate impurities when analyzing a sample or to
isolate an analyzer from the main sample stream.
FIG. 4 illustrates an alternate flow control device;
FIG. 5 illustrates an alternate form of the invention; and
FIG. 6 illustrates an application of the present invention.
DESCRIPTION OF THE PRESENT INVENTION
Referring now to FIG. 1, the permeand container of the present invention
includes a tubular housing member 10 made of aluminum which is internally
threaded at both ends to receive aluminum end caps 11,12. By way of
illustration, the housing member 10 can have an outer diameter of 1 inch,
a wall thickness of 1/16 inch and a length of 6 inches to provide a
container volume of 2.6 cubic inches. In the lower end is the cap member
12 that has a flange which cooperates with a fluorocarbon seal member 13
and the housing member 10 to provide a compression gas tight seal. The cap
member 12 is tightly fitted into the counterbore in the housing member. A
central port 15 through the end cap 12 has an enlarged threaded bore
section to receive an inlet seal plug 16 constructed from fluorocarbon and
an inlet set screw or closure member 17. The port 15 has a second enlarged
bore section to receive an elastomer sealing disc 18 and a tubular bushing
clamp 19. The interior of the cylinder case 10 can be filled with a
liquified gas such as sulfur dioxide, where this trace gas is desired, by
hypodermic injection through the sealing disc 18.
At the upper end of the housing member 10 is the cap member 11, which is
tightly received in a counterbore and which cooperates with a fluorocarbon
seal to provide a gas tight seal. The cap member 11 has an extending
tubular outlet section 22 which has an exterior configuration for
attachment to a tee coupling in a main gas line. The outlet section 22 has
an elongated central bore 23 which joins an enlarged counterbore 24 in the
cap member 11. The enlarged counterbore 24 receives a cylindrically shaped
elongated permeation member 25 constructed of a permeable material. The
permeation member has an upper bore portion 26, sized to receive a seal
pin, which continues the bore 23 of the outlet section and has a lower
bore portion 27 with reduced diameter which receives permeate. The lower
bore portion 27 is a blind bore which extends lengthwise of the permeation
member 25 from a location near the interior end surface of the end cap 11.
The permeation member 25 may be constructed from one of various polymeric
materials chosen for the permeation rate desired and for chemical
inertness to the permeand and for minimum change in rate with temperature.
The permeation member 25 serves to provide a controlled permeation of the
permeand in the housing member to a permeate in the bore 27 and the
permeate is utilized for introduction as a trace fluid in another liquid
or stream of gas. In the permeation member 25, the length of the bore 27,
its cross sectional area, the pressure within the housing 10, the pressure
within the bore and the material of the permeation member 25 affect the
permeation rate. The length of the bore 27 between the end of the end cap
11 and the end of the blind bore 27 is herein defined as the permeation
zone. The permeation zone has a length and diameter which determines a
permeation rate or output flow rate. The length of the permeation zone and
its diameter are grossly determined with respect to a permeation member
and a permeand by empirical methods. The permeation member 25 may, for
example, extend into the container for a length of 2 inches with a bore
diameter of 0.082 inches for chlorene as a permeand and FEP fluorocarbon
as the material for the permeation member 25.
The bore 26 can be sealed for shipment or for storage by using a stainless
steel wire element 30 which is sized to plug off the bore 26. The wire
element 30 is subsequently removable by cooling the housing sufficiently
to reduce the internal pressure below the pressure which would fracture
the permeation member prior to removing the wire element 30.
To minimize diffusion of carrier gas from a main gas line into the bore 27,
which could effect permeation rate, a longitudinally curved stainless
steel filler rod 32 is insertable into the bore 23. The diameter of the
rod 32 is less than the diameter of the bore 23 and serves to restrict the
size of the bore 23 and provide an annulus output path so that the
permeate in the annulus has sufficient flow to prevent back diffusion of
the carrier gas into the bore 27. Pulsations caused by carrier pressure
changes are minimized.
As described above, the bore 27 in the permeation member 25 is sized and
drilled and then tested to determine the permeation rate for a given
permeation material and permeand. If the output permeation rate is
insufficient for the surface area of the bore diameter and length, a
deeper or longer bore 27 can be drilled, even after filling the housing
with a permeand. Alternatively, the bore 27 can be redrilled to a larger
diameter to increase the surface area. The filler rod 32 can reduce the
annular space to less than 0.001 inch which also allows the permeate to
flush out the exit bore 23 in use. In the even that the permeation rate
for a given bore length and bore diameter is too great or greater than
desired, the flow rate can be decreased by inserting a solid ball into the
bore to decrease the effective surface area for permeating. As shown in
FIG. 4, a rod 50 with an attached ball member 51 can be utilized as a
control for the permeation rate and a desired permeation rate is obtained
by adjustment of the ball position in the bore 27. The rod 50 also serves
as a back diffusion limiting means.
Referring now to FIG. 2, another form of invention is illustrated. In FIG.
2, the outlet section 22 of a permeand filled housing or container 10 is
coupled to the input of a T coupling 35. The T coupling 35 has a tapered
input bore 36 which receives a seal gland 37 and the output end of the
outlet section 22. A coupling nut 38 sealingly connects the outlet section
22 in the bore 36. A dip tube 40 is inserted into the bore 23,27 of the
outlet section 22 so that carrier fluid flow from inlet 42 is directed
into the dip tube 40, the open end of the dip tube 40 being sealed off in
the inlet 42 by a seal gland 44. Carrier fluid flow in the dip tube 40 is
carried to a location near the bottom of the bore 27 in the permeation
member and passes upwardly through the annulus between the dip tube 40 and
the bore 23,27 in the permeation member to the output passage 46 of the T
coupling. This method sweeps out permeate reducing its partial pressure in
bore 27 to almost zero and insures a driving force equal to the full vapor
pressure of the permeand to increase the pressure differential across the
wall of the permeation member 25.
In apparatus as shown in FIG. 2 and as described hereafter in FIG. 3, it is
sometimes desirable to develop a permeate in the bore which has a partial
pressure below atmospheric pressure. In such instance condensation can
occur, however, by use of the carrier fluid as a sweeping agent across the
surface area of the bore, the partial pressure of the permeate in the bore
can be reduced to essentially zero so that the differential pressure
across the wall of the permeation member is optimized. Without the dip
tube sweep, liquid having vapor pressure below atmospheric would cease to
permeate when vapor pressure on both sides of the barrier, reaches
equalibrium.
Referring now to FIG. 3, in this embodiment of the present invention, the T
coupling and dip tube arrangement are the same as described with respect
to FIG. 2. However, the housing 10 is provided with an input opening 47
which is connected to a source of permeand. The output opening 48 allows
venting to remove impurities that could concentrate and alter rate of
flow. Gases such as carbon monoxide, nitrogen and the like are not easily
liquified. However, with the present invention, a gas such as nitrogen can
be throughput through the chamber 50; in gas form and not require
liquification. In this case, the driving force or pressure on the
permeation member 25 is the pressure of the permeand gas within the
chamber 50 in the housing 10. The gaseous permeate output in the bore
23,27 is controlled by controlling the pressure of the permeand gas in the
chamber 50. If the permeand gas is pure it is not necessary to have an
output flow of permeand gas from the chamber 50.
In another aspect of the present invention illustrated in FIG. 3, some
constituents in a process stream are undesirable when the process stream
is subjected to analysis. The structure shown in FIG. 3 can be utilized to
filter out impurities by passing the process stream through the chamber 50
while passing clean carrier gas through the dip tube 40. In this manner
the impurities are effectively blocked by the permeation member while the
permeate is transferred into a clean carrier gas in the output passage 46.
In the case of hydrogen sulfide, a reduction in concentration of the
hydrogen sulfide in the carrier gas results when high flow rates of
carrier gas are used and is a considerable benefit because some analyzers
operate best at low hydrogen sulfide concentration.
An alternate form of the invention is shown in FIG. 5 where the permeation
member 25a extends into a bore in the bottom wall 60 of the housing 10.
Gases or permeand in the housing chamber 50 permeate through the
permeation member 25a while carrier fluid is passed through the bore 62 of
the permeation member.
In FIG. 6, an application of the present invention is illustrated where a
dispenser 70 with a predetermined permeate volumetric output per unit of
time is connectable to a fixed volume tank 72 which contains a liquid or
gas to receive a fixed or predetermined amount of permeate from the
dispenser. In contra distinction to prior systems, the dispenser is
connected to the tank for a fixed period of time which determines the
concentration. For example, if the dispenser disperses 41,790
nanograms/minute of H.sub.2 S into a fixed volume of 500 milliliters the
concentration within the container will increase by 1 part per million
(PPM) per second. Thus, in 10 seconds of time, the concentration produced
is 10 PPM by volume. The equation is:
n=(x)(y)(83.58) (1)
where
x is container volume in milliliters;
y is parts per million per second desired; and
n is nanograms per minute needed.
As shown in FIG. 6, the mixing of the concentration can be enhanced by use
of a pump 74 to circulate the mixture during injection.
It will be apparent to those skilled in the art that various changes may be
made in the invention without departing from the spirit and scope thereof
and therefore the invention is not limited by that which is disclosed in
the drawings and specifications but only as indicated in the appended
claims.
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