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Device and Method for Producing Sodium Hydride

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Device and Method for Producing Sodium Hydride Europäisches Patentamt *EP001277700A2* (19) European Patent Office Office européen des brevets (11) EP 1 277 700 A2 (12) EUROPEAN PATENT APPLICATION (43) Date of publication: (51) Int Cl.7: C01B 6/04, B01J 19/08 22.01.2003 Bulletin 2003/04 (21) Application number: 02077752.0 (22) Date of filing: 09.07.2002 (84) Designated Contracting States: (72) Inventors: AT BE BG CH CY CZ DE DK EE ES FI FR GB GR • Agnew, Stephen F. IE IT LI LU MC NL PT SE SK TR San Diego, California 92117 (US) Designated Extension States: • Putvinski, Sergei AL LT LV MK RO SI La Jolla, California 92037 (US) (30) Priority: 10.07.2001 US 903321 (74) Representative: Gates, Marie Christina Esther et al (71) Applicant: Archimedes Technology Group, Inc. c/o Tomkins & Co. San Diego, CA 92121 (US) 5 Dartmouth Road Dublin 6 (IE) (54) Device and method for producing sodium hydride (57) A device and method for producing sodium the plasma torch, the plasma jet is introduced into a (Na) from a feed material such as a mixture of methane chamber where a magnetic field has been established. (CH4) and sodium hydroxide (NaOH) includes a plasma Once inside the chamber the heated mixture of ions and torch configured to heat the feed material to a temper- neutrals interacts with the magnetic field in the chamber ature sufficient to reduce and ionize sodium (Na). As to cause the sodium ions to travel substantially along such, a plasma jet is created by the plasma torch that the magnetic field lines while the neutrals travel on paths contains ionized sodium (Na) and non-ionized neutrals that are essentially unaffected by the magnetic field. A such as hydrogen (H) and carbon monoxide (CO). From collector is positioned to receive and accumulate sodi- um (Na). EP 1 277 700 A2 Printed by Jouve, 75001 PARIS (FR) EP 1 277 700 A2 Description FIELD OF THE INVENTION 5 [0001] 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. 10 BACKGROUND OF THE INVENTION [0002] 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, 15 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: 20 NaH + H20 ⇒ NaOH + H2. (Reaction 1) [0003] For example, U.S. Patent No. 5,728,464 entitled "Hydrogen Generation Pelletized Fuel" which issued to 25 Checketts on March 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. [0004] 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 30 sodium hydroxide (NaOH) in a methane (CH4) atmosphere. Specifically, at a reaction temperature of approximately 900 C (1173 K), the reaction: NaOH + CH4 ⇒ CO + 2.5 H2 + Na(g) (Reaction 2) 35 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(I) condenses on the collector. Specifically, in accordance with the following analysis, pressures exceeding approximately 4200 atmospheres are required to condense liquid sodium Na(I) rather than sodium hydroxide (NaOH) on the cold collector. 40 [0005] 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 ap- pears to be futile. Specifically, the following compounds (all in the gas phase) have been considered; H, Na, O, Na+, H2,O2, OH, NaH, CO, NaO, NaOH, CO2,H2O, H2CO, CH4. 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 45 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. [0006] Referring to Fig. 1, it can be seen that at low temperatures, T < 600 K, the major components are NaOH and CH4. In the temperature range, 1000 K < T < 2000 K, the major compounds are indeed CO, H2, and Na vapor as predicted by Reaction 2. At T > 3000 K, Na becomes ionized and H2 molecules dissociate. The other compounds 50 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: 55 pCH4 = pNaOH, pCO = pNa, pH2 = 2.5pNa. (eq. 1) 2 EP 1 277 700 A2 As such, the total pressure will be: P = pNaOH + pCH4 + pNa + pCO + pH2 = 4.5pNa + 2pNaOH. (eq. 2) 5 Thus, the equilibrium equation for Reaction 2 can be written as follows: 2.5 (pNapCOpH2 )/(pNaOHpCH4) = K(T) 10 or using equation (1): 2.5 4.5 2 2.5 pNa /pNaOH = K(T). (eq. 3) 15 [0007] 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. 20 [0008] 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 H2, CO and CH4 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 (psat = p) for NaOH occurs at a higher 25 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 pres- sures: 30 2.5 4.5 2 2.5 ps,Na /psNaOH < K(T). [0009] The above condition is satisfied at T > 3000 K and a total pressure of approximately: 35 p > 4.5ps,Na + 2ps,NaOH = 4200 atm which is simply not practical. Thus, the above analysis indicates that at moderate pressures, the equilibrium conden- 40 sation of Na does not take place. [0010] The present invention contemplates separation of Na from the other gases by ionization. For example, con- sider a mixture of NaOH and CH4 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 H2) by introducing 45 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. 50 [0011] First, consider a comparison between the magnetic pressure and the gas pressure. Magnetic pressure, pm, can be found using the equation: 2 pm = B /8π, 55 or in practical units 3 EP 1 277 700 A2 2 pm [Pa] = BG /80π. 4 For example, for B = 3 kG, pm = 3.6 10 Pa = 270 Torr which is larger then the expected gas pressure in the plasma 5 jet, p = 1-5 kPa.
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