Electric heating – Metal heating – By arc
Reexamination Certificate
2001-07-10
2003-09-23
Paschall, Mark (Department: 3742)
Electric heating
Metal heating
By arc
C219S121590, C219S121430, C075S745000
Reexamination Certificate
active
06624380
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains generally to devices and methods for chemical separation. More particularly, the present invention pertains to devices and methods for the extraction of sodium from sodium compounds. The present invention is particularly, but not exclusively, useful for recovering sodium hydride from a mixture of methane and sodium hydroxide.
BACKGROUND OF THE INVENTION
Recently, there has been an abundant interest in the use of sodium hydride (NaH) as a portable energy source to produce hydrogen as a replacement for fossil fuels such as gasoline. For example, engines similar to the standard gasoline engines that are now used in automobiles can be manufactured that use hydrogen gas as a fuel. Unfortunately, the bulk amount of actual hydrogen gas that is needed for vehicle applications would require either an extremely large tank or a high-pressure vessel. Both of these requirements can be expensive and impractical for use on a vehicle. Thus, to avoid these requirements, devices have been proposed to produce hydrogen gas at relatively low pressures. Importantly, the gas can be produced as demanded by the hydrogen engine, by reacting sodium hydride with water according to the reaction:
NaH+H
2
O
NaOH+H
2
(Reaction 1)
For example, U.S. Pat. No. 5,728,464 entitled “Hydrogen Generation Pelletized Fuel” which issued to Checketts on Mar. 17, 1998 discloses sodium hydride pellets for a hydrogen demand system. Specifically, Checketts discloses sodium hydride pellets that are coated with a water impervious barrier. The barrier can be removed either mechanically or electrically to expose the sodium hydride core for reaction with water to produce hydrogen.
As indicated by Reaction 1 above, a by-product of the reaction is sodium hydroxide (NaOH). It has been proposed elsewhere to recover sodium hydride (NaH) from the by-product sodium hydroxide (NaOH) by heating the sodium hydroxide (NaOH) in a methane (CH
4
) atmosphere. Specifically, at a reaction temperature of approximately 900 C (1173 K), the reaction:
NaOH+CH
4
CO+2.5H
2
+Na(
g
) (Reaction 2)
can be used to produce sodium gas Na(g). Unfortunately, when the hot, reaction products of Reaction 2 are sent to a cold collector under modest pressures, sodium hydroxide (NaOH) rather than liquid sodium Na(l) condenses on the collector. Specifically, in accordance with the following analysis, pressures exceeding approximately 4200 atmospheres are required to condense liquid sodium Na(l) rather than sodium hydroxide (NaOH) on the cold collector.
Before concluding that impractical pressures are required to condense liquid sodium, attempts to shift the equilibrium by introducing other compounds into the mixture were considered. After consideration, this approach appears to be futile. Specifically, the following compounds (all in the gas phase) have been considered; H, Na, O, Na+, H
2
, O
2
, OH, NaH, CO, NaO, NaOH, CO
2
, H
2
O, H
2
CO, CH
4
. Carbon has very low vapor pressure and, therefore, carbon vapor has been excluded from the above list.
FIG. 1
shows the concentrations of the different compounds as a function of temperature at a total pressure of 1 atm.
FIG. 1
was obtained theoretically by minimizing free energy and using balance equations for the different elements. Compounds having a concentration less then 10
−8
are not shown.
Referring to
FIG. 1
, it can be seen that at low temperatures, T<600 K, the major components are NaOH and CH
4
. In the temperature range, 1000 K<T<2000 K, the major compounds are indeed CO, H
2
, and Na vapor as predicted by Reaction 2. At T>3000 K, Na becomes ionized and H
2
molecules dissociate. The other compounds considered and listed above are not essential. Atomic and molecular oxygen is not present in the full temperature range. Therefore, at low temperatures, when Na is not ionized and hydrogen is in molecular form, a simple model based on Reaction 2 can be used. The partial pressures of methane, carbon monoxide and hydrogen can be expressed in terms of the partial pressures of NaOH and Na, using Reaction 2 as follows:
p
CH4
=p
NaOH
, p
CO
=p
Na
, p
H2
=2.5
p
Na
(eq. 1)
As such, the total pressure will be:
p=p
NaOH
+p
CH4
+p
Na
+p
CO
+p
H2
=4.5
p
Na
+2
p
NaOH
(eq. 2)
Thus, the equilibrium equation for Reaction 2 can be written as follows:
(
p
Na
p
CO
p
H2
2.5
)/(
p
NaOH
p
CH4
)=
K
(
T
)
or using equation (1):
2.5
2.5
p
Na
4.5
/p
NaOH
2
=K
(
T
) (eq. 3)
Equations 2 and 3 allow the partial pressures of Na and NaOH to be evaluated as function of total pressure, p, and temperature, T. Thus, the full model revealed by
FIG. 1
comports closely with the simple model (Reaction 2) at low temperatures, T<2000 K. Further, equations 2 and 3 show that at higher pressures, higher temperatures are required to reduce sodium.
Next, an analysis can be conducted to determine the temperature range in which the gaseous model is valid. Specifically, the gaseous model is valid when the partial pressures of Na or NaOH are less then the saturated pressures for these compounds. The other major compounds such as H
2
, CO and CH
4
have very high vapor pressures, and accordingly, do not condense.
FIG. 2
shows the saturated pressure to partial pressure ratios for the case presented in FIG.
1
. It can be seen that at a total pressure of p=1 atm, the condensation point (p
sat
=p) for NaOH occurs at a higher temperature than for Na. Thus, at this pressure, Na will be collected in the form of NaOH rather then metallic Na. An increase in the total pressure can shift the reaction and in principle can create a condition where Na has a condensation point at a higher temperature than NaOH. The total pressure necessary to condense Na rather than NaOH can be derived from equation 3 by replacing the partial pressures of each constituent by their corresponding saturated pressures:
2.5
2.5
p
s,Na
4.5
/p
sNaOH
2
<K
(
T
)
The above condition is satisfied at T>3000 K and a total pressure of approximately:
p>
4.5
p
s,Na
+2
p
s, NaOH
=4200
atm
which is simply not practical. Thus, the above analysis indicates that at moderate pressures, the equilibrium condensation of Na does not take place.
The present invention contemplates separation of Na from the other gases by ionization. For example, consider a mixture of NaOH and CH
4
heated to a temperature of 3000-4000 K rather than to 1000 K. This heating can be accomplished using a plasma torch. At these higher temperatures, Na atoms will be fully ionized. The present invention further contemplates separating the ionized Na component from the non-ionized neutrals (i.e. CO and H
2
) by introducing the mixture in the form of a plasma jet into a strong magnetic field. In the magnetic field that is directed along the jet, sodium ions will move predominantly along the magnetic field lines and neutrals will diffuse from the plasma jet radially, where the neutrals can be pumped from the device. As such, an increase of sodium concentration along the plasma jet can be expected. Specifically, the following analysis estimates the increase in sodium concentration along the plasma jet.
First, consider a comparison between the magnetic pressure and the gas pressure. Magnetic pressure, p
m
, can be found using the equation:
p
m
=B
2
/8&pgr;,
or in practical units
p
m
[Pa]=B
G
2
/80&pgr;.
For example, for B=3 kG, p
m
=3.6 10
4
Pa=270 Torr which is larger then the expected gas pressure in the plasma jet, p=1-5 kPa. To derive the radial velocities of the neutrals, ions, and electrons, momentum balance equations for these particles with friction forces acting between different components can be considered. Assuming a cylindrical plasma jet in a uniform axial magnetic field, the result is:
V
ri
=V
re
=−(
c/eB
)
2
(
dp
&Sgr;
/dr
)(&mgr;
i0
&mgr;
e0
K
i0
K
e0
n
0
i
/(&mgr;
i0
K
i0
&
Agnew Stephen F.
Putvinski Sergei
Archimedes Technology Group, Inc.
Nydegger & Associates
Paschall Mark
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