Recycle of discharged sodium borate fuel

Details Recycle of discharged sodium borate fuel Recycle of discharged sodium borate fuel

2003-05-12

2004-03-16

McKane, Joseph K. (Department: 1626)

Organic compounds -- part of the class 532-570 series

Organic compounds

Borate esters

Reexamination Certificate

active

06706909

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to processes for enhancing the recovery of boron values from alkali metal borates.
BACKGROUND OF THE INVENTION
Environmentally friendly fuels, e.g., alternatives to hydrocarbon based energy sources, are currently of great interest. Borohydride salts are an example of compounds which can be used directly as an anodic fuel in a fuel cell or as a hydrogen storage medium to enable hydrogen fuel cell applications. As a hydrogen storage material, alkali metal borohydrides produce hydrogen by a hydrolysis reaction, which also produces alkali metal borate salts as a byproduct. Further, when aqueous solutions of borohydride compounds are used as the hydrogen storage medium, it is preferable to add an alkaline stabilizing agent, typically an alkali metal hydroxide compound, to the mixture. This stabilizing agent is present unchanged in the discharged borate solution. In addition to the need to manufacture borohydride compounds in an energy efficient manner, there is a corresponding need to dispose of the large quantities of alkali metal borate salts that would result from extensive use of a borohydride-based fuel system, particularly given the quantities thereof that would be consumed, e.g. by the transportation industry. It will be appreciated that a particularly advantageous means of disposing of the alkali metal borates would be via a process that converts them back into borohydride compounds.
Processes are known whereby borate salts can be converted into trialkyl borates, which are the precursors in the current commercial process for the synthesis of borohydride. Preferred among these is the formation of trialkyl borate compounds which involves two steps. The first step is the acidification of the borate compound, e.g. borax, with an acid, primarily a strong acid such as sulfuric acid, to form boric acid and the alkali metal sulfate, as shown in Equation (1) wherein the alkali metal is sodium:
 Na
2
B
4
O
7
+H
2
SO
4
+5H
2
O→4B(OH)
3
+Na
2
SO
4
  (1)
The second step is the reaction of the resultant boric acid with a lower alcohol to form the desired trialkyl borate as shown in Equation (2):
B(OH)
3
+3ROH→B(OR)
3
+3H
2
O  (2)
Wherein R is a saturated lower alkyl group
The trialkyl borate may then be converted to borohydride by a process shown in Equation (3), wherein Y is an alkali metal, preferably sodium.
B(OR)
3
+4YH→YBH
4
+3 YOR  (3)
The process of forming trialkyl borates from alkali metal borates, such as borax, by the reactions shown in Equations (1) and (2) is disadvantageous in three aspects. The first is the generation of large amounts of alkali metal sulfate, typically sodium sulfate that must be disposed of. Second, the fact that boric acid is formed in a separate step reduces the recovered boron value per pass as a result of limitations of the crystallization process. Finally, the water formed in the alcohol reaction shown in Equation (2), in combination with the desired trialkyl borate, can enter into an equilibrium with a reverse reaction back to boric acid which retards the reaction rate and, ultimately, can reduce the yield of the trialkyl borate.
Another method of obtaining trialkyl borates is by the direct acidification of alkali metal borate salts or borate ore with carbon dioxide in the presence of a lower alcohol at elevated temperatures as taught, for example, in Canadian Patent No. 624,125. This reaction, illustrated with methanol as the lower alcohol, is shown in Equation (4).
NaBO
2
+CO
2
+3CH
3
OH→B(OCH
3
)
3
+NaHCO
3
+H
2
O  (4)
In this reaction, there is also an equilibrium in the reaction chamber between boric acid and trimethyl borate as discussed above. In this instance, both are recovered as products, boric acid by recognized crystallization techniques and trimethyl borate by distillation techniques. It is stated that the reaction is advantageously conducted in a grinding apparatus, such as a ball mill, to maintain the borate substrate material in a fine state of subdivision, thereby facilitating maximum contact among the reactants. It is also stated that the reaction is advantageously conducted at higher temperatures when the substrate borate material contains water of hydration since the higher temperature facilitates dehydration of the substrate. It is not stated how the water is removed from the reaction chamber other than the equilibrium reaction discussed above. In this instance, both the boric acid and trimethyl borate are recovered. Trimethyl borate yields of 16% are disclosed for sodium metaborate as a starting material at 500° C. and up to 67% for borax as a starting material at 600°C.
In U.S. Pat. No. 2,884,440, there is disclosed processes for the conversion of borax to trimethyl borate by the reaction with carbon dioxide and methanol at temperatures between 0° C. and 195° C. and pressures between 1 atm and 35 atm, which represents about 550 psig. The reaction is shown is Equation (5)
Na
2
B
4
O
7
.xH
2
O+CO
2
+nCH
3
OH→4B(OCH
3
)
3
+(x+6)H
2
O+(n−12)CH
3
OH+Na
2
CO
3
  (5)
wherein x represents the number of moles of water of hydration of the sodium tetraborate (10 in the instance of borax) and n represents the number of moles of methanol added to the reaction mixture. It is disclosed that the sodium carbonate formed may, in turn, react with carbon dioxide and water present to form sodium bicarbonate. It is stated that both sodium carbonate and sodium bicarbonate can be calcined to recover carbon dioxide for use in the process.
The reactions shown in Equations (4) and (5) are important in that they demonstrate that the first step in the recovery of alkali metal borate can be carried out without the need to utilize a strong acid with the inherent danger of an uncontrolled hyperthermia that might result from a reaction with sodium hydroxide which is typically present in discharged fuel from a hydrogen generation, apparatus. These reactions further demonstrate that, in producing trialkyl borates, there is significantly less difficulty in converting borax than sodium metaborate, which would be expected since borax is the more acidic of the two. However, these reactions are also limited by the accumulation of water in the reaction medium. Here also, the water tends to form an equilibrium with boric acid that will adversely affect both the rate of reaction and the yield of trialkyl borate as discussed above.
In view of the large volume of use contemplated for borohydride as a fuel, e.g., in the transportation industry, there is clearly a need for a process of recovering borohydride for future use that is cost-effective and environmentally acceptable. In order to meet these criteria, the process should readily separate boron from any sodium species present, avoid the use of strong acid and thereby prevent the danger of runaway hyperthermia, provide a boron species that is directly reducible to a boron hydride, and recover boron values in a cost-effective manner. It is further important that the process be carried out directly on the discharged fuel without the need to first separate the boron from the alkali metal hydroxide stabilizer. Such a process is provided in accordance with the present invention.
SUMMARY OF THE INVENTION
In accordance with the present invention, a process is provided for recovering boron value from a mixture of alkali metal hydroxide and alkali metal borate that represents the discharged fuel from a hydrogen generation apparatus. The process comprises initially dehydrating the discharged fuel solution, then treating it with carbon dioxide and a lower alcohol, preferably methanol, to form the corresponding trialkyl borate, alkali metal bicarbonate and water. The formation of the trialkyl borate is improved upon in accordance with the present invention by the presence of a solid, porous water-absorbing material in the reaction mixture to absorb water as it is formed, thus minimizing the formation of bori

Affiliated with

Also associated with

Rating

Recycle of discharged sodium borate fuel does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Recycle of discharged sodium borate fuel, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Recycle of discharged sodium borate fuel will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3222433